Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 29 November-5 December seismicity at Nevado del Ruiz was characterized by a decrease in the number and magnitude of earthquakes compared to the previous week. Significant amounts of water vapor and gas rose from the crater. On 2 December a gas, water vapor, and ash plume rose 2 km above the crater rim and drifted between SW and NW directions. The Alert Level remained at III (Yellow; the second lowest level on a four-color scale).

During September 2012-July 2014 the Servicio Geológico Colombiano (SGC) maintained Alert Level Yellow at Nevado del Ruiz due to elevated SO2 flux, seismicity, and intermittent ash emissions; the Alert Level had been lowered to Yellow on 5 September. No major geophysical changes were noted during this time period.

Gas plumes were frequently visible rising as high as 3,500 m above the summit (figure 1). Minor gas-and-ash plumes occasionally occurred throughout this reporting period, and were often associated with episodes of tremor. Ashfall was reported from areas around the volcano (figure 2), notably on 11 July 2013 when ash reached Manizales (~30 km NW). The Washington Volcanic Ash Advisory Center (VAAC) reported ash plumes visible in satellite images on 10 October 2012 (reaching ~7 km altitude) and 11 July 2013 (reaching ~6 km altitude). A possible ash plume was detected at ~7.6 km altitude on 16 June 2013.

Figure 2. Persistent ashfall challenged the local monitoring systems for Nevado del Ruiz in September 2012. These two stations, a ScanDOAS station Bruma/Azufrado (left) and the GPS station at Gualí (right), were the focus of fieldwork by SGC staff in September. Courtesy of SGC.

SGC reported continuous and elevated SO2 emissions during this reporting period. Gas monitoring included satellite images, field installations (ScanDOAS), and MobileDOAS. ScanDOAS values captured the dramatic increase in SO2 flux during the explosive eruptions of early 2012 (BGVN 37:08). Since September 2012, the maximum daily average frequently exceeded 6,000 tons per day and exceeded 9,500 tons/day three times: twice in June 2013 and once in July 2014. MobileDOAS measurements were collected by the SGC during field campaigns and the values are reported in monthly technical bulletins available online (in Spanish).

Seismicity. Volcano-tectonic earthquakes (VT), long-period earthquakes (LP), and tremor were detected by the monitoring network throughout this reporting period. Earthquakes were detected from the summit glaciers, as well, notably in February 2014. Increased VT seismicity was notable during October 2012 (figure 3), 23-24 February 2013, 13 April-4 May 2013, 5-10 October 2013, 18-27 June 2014, and 29 July 2014 (figure 4). Maximum local magnitudes of VT earthquakes in the range of 1.6-4.4 were detected during this time period at depths of 0.5-13 km.

Figure 3. Seismicity from Nevado del Ruiz during October 2012 was elevated; 2,179 VT earthquakes were located. Two main areas of concentration were located N and NE of the summit (circled in red). Black squares are seismic stations; color coding refers to depth; circle size correlates with earthquake magnitudes. Courtesy of SGC.

Figure 4. Elevated seismicity from Nevado del Ruiz on 29 July 2014 consisted of 746 VT earthquakes detected in a zone ~2 km long SSW of the main crater, Arenas. Red lines correspond to local faults as well as Palestina, the regional, cross-cutting fault trending NE-SW. Depths of earthquakes are correlated to colors in the key as well as magnitudes based on circle size. Courtesy of SGC.

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 29 November-5 December seismicity at Nevado del Ruiz was characterized by a decrease in the number and magnitude of earthquakes compared to the previous week. Significant amounts of water vapor and gas rose from the crater. On 2 December a gas, water vapor, and ash plume rose 2 km above the crater rim and drifted between SW and NW directions. The Alert Level remained at III (Yellow; the second lowest level on a four-color scale).

Based on information from the Bogota MWO, the Washington VAAC reported on 16 November an ash plume from Nevado del Ruiz rose to an altitude of 6.7 km (22,000 ft) a.s.l. and drifted NE. Weather clouds prevented satellite views.

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that seismic signals indicating ash emissions from Nevado del Ruiz were recorded at 0608 on 27 October and 1341 on 30 October. The ash plume from the 27 October event drifted NE; weather clouds prevented conformation of the second event. The Alert Level remained at III (Yellow; the second lowest level on a four-color scale).

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 18-24 October seismicity at Nevado del Ruiz was characterized by a slight decrease in the number and magnitude of earthquakes compared to the previous week. Some seismic signals were associated with gas-and-ash emissions which were confirmed by webcam images, and Parque Nacional Natural los Nevados (PNNN) and SGC officials. A steam, gas, and ash plume rose 2 km above the crater and drifted W on 18 October. Two low-energy thermal anomalies were detected on 18 and 20 October. The Washington VAAC reported that on 19 and 20 October ash plumes rose to altitudes of 7.3 and 6.1 km (24,000 and 20,000 ft) a.s.l. and drifted NW and SW, respectively. The Alert Level remained at III (Yellow; the second lowest level on a four-color scale).

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 11-17 October seismicity at Nevado del Ruiz was characterized by a decrease in the number and magnitude of earthquakes compared to the previous week. Gas-and-ash plumes were visible on 12 and 13 October. On 15 October gas, steam, and ash plumes rose as high as 2.5 km above the crater and drifted NW. The number of long-period events increased during 1800-2300 on 17 October; some of the signals were associated with gas-and-ash emissions. Significant amounts of water vapor and gas rose from the crater. The Alert Level remained at III (Yellow; the second lowest level on a four-color scale).

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 4-10 October the number and magnitude of volcano-tectonic earthquakes at Nevado del Ruiz increased compared to the previous week. Long-period and very-long-period signals as well as pulses of tremor were also recorded. Significant amounts of water vapor and gas rose from the crater. At 1746 on 5 October an episode of volcanic tremor was associated with ash emissions recorded by area webcams. Gas, steam, and ash plumes rose 2 km above the crater on 8 October and drifted NE. The Alert Level remained at III (Yellow; the second lowest level on a four-color scale).

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that a webcam recorded an ash plume rising from Nevado del Ruiz at 0558 on 29 September. Based on information from the Bogota MWO, the Washington VAAC reported on the same day that an ash plume rose to an altitude of 6.1 km (20,000 ft) a.s.l. and drifted WSW. Later that day ASHTAM reports indicated an ash emission to an altitude of 8.5 km (28,000 ft) a.s.l. and drifted 13 km S, though cloud cover prevented webcam and satellite image views. An ash emission reported by the Bogota MWO rose to an altitude of 6.1 km (20,000 ft) a.s.l. and drifted NE on 3 October. The Alert Level remained at III (Yellow; the second lowest level on a four-color scale).

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 20-26 September seismicity at Nevado del Ruiz was characterized by an increased number of earthquakes and a slight decrease in the seismic energy compared to the previous week. Significant amounts of water vapor and gas rose from the crater. Gas, steam, and ash plumes rose 700 m above the crater on 21 September and drifted NW. The Alert Level remained at III (Yellow; the second lowest level on a four-color scale).

Based on satellite images, the Washington VAAC reported that ash emissions at Nevado del Ruiz began at 0615 on 13 September, producing a plume that rose to an altitude of 6.1 km (20,000 ft) a.s.l. and drifted W. By 1345 the ash had dissipated.

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 23-29 August seismicity at Nevado del Ruiz was relatively unchanged compared to the previous week. Significant amounts of water vapor and gas rose from the crater. Ash emissions occurred on several days, and sometimes several times per day. Based on notices from the Bogota MWO, satellite and webcam images, and model data, the Washington VAAC reported that during 2-3 September ash plumes rose to altitudes of 6.1-6.4 km (20,000-21,000 ft) a.s.l. and drifted W and NW. SGC noted that gas, steam, and ash plumes occasionally rose 1.8 km above the crater rim and drifted NW and W on 4 and 6 September. The Alert Level remained at III (Yellow; the second lowest level on a four-color scale).

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 23-29 August seismicity at Nevado del Ruiz slightly increased compared to the week before. Significant amounts of water vapor and gas rose from the crater. Thermal anomalies were detected on 24 and 29 August. A period of tremor that started at 1747 on 26 August was associated with an ash emission that rose 900 m above the crater. A gas-and-ash plume, associated with a seismic signal at 1050 on 27 August, was confirmed by a pilot. Based on webcam images and seismic signals, an ash plume rose 1.3 km and drifted NW on 28 August. Gas, steam, and ash plumes occasionally rose 2.3 km above the crater rim and drifted NW and W on 29 August. The Alert Level remained at III (Yellow; the second lowest level on a four-color scale).

Based on information from the Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales, the Washington VAAC reported that on 19 August an ash plume from Nevado del Ruiz rose to an altitude of 6.9 km (22,000 ft) a.s.l. and drifted W and SW. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 2-8 August seismicity at Nevado del Ruiz remained at similar levels as the week before. Significant amounts of water vapor and gas rose from the crater. A gas, steam, and ash plume rose 850 m above the crater rim and drifted NW and W on 6 August. The Alert Level remained at III (Yellow; the second lowest level on a four-color scale).

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 26 July-1 August seismicity levels at Nevado del Ruiz slightly increased as compared to the week before. Significant amounts of water vapor and gas rose from the crater. A gas, steam, and ash plume rose 700 m above the crater rim and drifted NW and W on 29 July. The Alert Level remained at III (Yellow; the second lowest level on a four-color scale).

Based on notices from the Bogota MWO and model data, the Washington VAAC reported that on 24 July a possible ash plume from Nevado del Ruiz rose to an altitude of 7.2 km (23,000 ft) a.s.l. On 26 July an ash plume recorded by the webcam and identified in satellite images rose to an altitude of 6.9 km (22,000 ft) a.s.l. and drifted W. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 12-18 July seismicity at Nevado del Ruiz slightly increased as compared to levels the week before. Several episodes of “drumbeat” earthquakes were recorded during 13, 15, and 16-17 July. These signals were low energy and indicated a growing lava dome. Significant amounts of water vapor and gas rose from the crater. Gas, steam, and ash plumes rose at most 500 m above the crater rim and drifted NW on 13, 14, 16, and 17 July. The Alert Level remained at III (Yellow; the second lowest level on a four-color scale).

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that a webcam recorded minor ash emissions rising from Nevado del Ruiz during 6-7 July. Based on notices from the Bogota MWO, satellite images, and model data, the Washington VAAC reported that on 8 July ash plumes rose to altitudes of 6.1-6.4 km (20,000-21,000 ft) a.s.l. and drifted W. A thermal anomaly was visible on 9 July. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 28 June-4 July seismicity at Nevado del Ruiz slightly decreased. Significant amounts of water vapor and gas rose from the crater during the week. A gas, steam, and ash plume rose 850 m above the crater rim and drifted NW on 28 June. The Alert Level remained at III (Yellow; the second lowest level on a four-color scale).

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that at 0822 on 27 June and at 0748 on 28 June episodes of volcanic tremor were associated with ash emissions that rose 1.8 and 0.9 km above Nevado del Ruiz, respectively. The emissions were confirmed by national park authorities and local residents. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 24-30 May seismicity at Nevado del Ruiz was characterized by long-period and very-long-period earthquakes, episodes of continuous tremor, and pulses of volcanic tremor associated with gas-and-ash emissions. Significant amounts of water vapor and gas rose from the crater during the week. A gas, steam, and ash plume rose 1.8 km above the crater rim and drifted NW and W on 25 May. A minor thermal anomaly near Arenas Crater was detected in satellite images on 27 May. The Alert Level remained at III (Yellow; the second lowest level on a four-color scale).

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 17-23 May seismicity at Nevado del Ruiz was characterized by long-period and very-long-period earthquakes, episodes of continuous tremor, and pulses of volcanic tremor associated with gas-and-ash emissions. Significant amounts of water vapor and gas rose from the crater during the week. Ashfall was reported in La Florida, Manizales, on 20 May. According to a news article an ash emission on 20 May prompted closure of the La Nubia airport in Manizales. Later that day a gas, steam, and ash plume rose 2.5 km and drifted NW and W. Based on information from SGC, the Washington VAAC reported that on 17 May an ash emission rose to an altitude of 7 km (23,000 ft) a.s.l. Cloud cover in the area prevented satellite observations of the activity. The Alert Level remained at III (Yellow; the second lowest level on a four-color scale).

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 10-16 May seismicity at Nevado del Ruiz was characterized by long-period and very-long-period earthquakes, episodes of continuous tremor, and pulses of volcanic tremor associated with gas-and-ash emissions. Thermal anomalies near Arenas Crater were identified in satellite images during 11-12 May. Significant amounts of water vapor and gas rose from the crater during the week. A gas, steam, and ash plume rose 1.7 km and drifted NW and W on 12 May. Based on information from SGC, the Washington VAAC reported that on 17 May an ash emission rose to an altitude of 7 km (23,000 ft) a.s.l. Cloud cover in the area prevented satellite observations of the activity. The Alert Level remained at III (Yellow; the second lowest level on a four-color scale).

Based on an SGC notice, the Washington VAAC reported that ash plumes from Nevado del Ruiz drifted over 10 km on 22 April and almost 20 km on 24 April. Possible ash emissions rose from the volcano on 25 April. Extensive weather clouds prevented satellite views on all three days.

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 12-18 April seismicity at Nevado del Ruiz was characterized by long-period earthquakes, episodes of continuous tremor, and pulses of volcanic tremor associated with gas-and-ash emissions. Thermal anomalies near Arenas Crater were periodically identified in satellite images. Significant amounts of water vapor and gas rose from the crater during the week. A gas, steam, and ash plume rose 1.5 km and drifted NW and NE on 12 April. The Alert Level remained at III (Yellow; the second lowest level on a four-color scale).

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that at 0804 on 31 March an episode of short-duration volcanic tremor was associated with an ash emission that rose 3.5 km above Nevado del Ruiz. According to the Washington VAAC the ash plume drifted NW and SW. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Based on an ASHTAM (a special Notice to Airmen for airborne ash), the Washington VAAC reported a possible emission from Nevado del Ruiz on 28 March. Ash was not identified in satellite images, though a thermal anomaly was detected. A period of increased seismicity was detected the next day, and observers reported another possible ash emission.

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that at 0606 on 18 March an episode of volcanic tremor was associated with an ash emission that rose 1.7 km above Nevado del Ruiz. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 8-14 March seismicity at Nevado del Ruiz was characterized by long-period earthquakes, episodes of continuous tremor, and pulses of volcanic tremor associated with gas-and-ash emissions. A gas, steam, and ash plume rose 1.8 km and drifted SW on 8 March. The next day an episode of volcanic tremor was associated with an ash emission recorded by the webcam and observed by National Park officials. Seismicity increased during 12-13 March. The largest earthquake was recorded at 0233 on 12 March; it was a local M 3, NW of Arenas Crater at a depth of 5.6 km. Significant amounts of water vapor and gas rose from the crater during the week. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

SGC reported that an episode of volcanic tremor at Nevado del Ruiz began at 0923 on 3 March and was associated with an ash plume that rose 3 km above the crater. Based on satellite and webcam images, the Washington VAAC stated that an ash plume rose to an altitude of 9.1 km (30,000 ft) a.s.l. (about 800 m higher than the SGC estimate) and drifted NW. Later that day satellite images detected an ash plume drifting 40 km W. According to the VAAC, the Bogota MWO reported that on 7 March an ash plume rose to an altitude of 6.1 km (20,000 ft) a.s.l. and drifted WNW.

Based on notices from the Bogota MWO, pilot observations, and wind data, the Washington VAAC reported that on 24 February an ash plume from Nevado del Ruiz possibly rose to altitudes of 5.5-6.1 km (18,000-20,000 ft) a.s.l. and drifted NW and S. According to the VAAC, SGC reported an ash emission on 29 February.

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 9-15 February seismicity at Nevado del Ruiz was characterized by long-period earthquakes and pulses of volcanic tremor associated with gas-and-ash emissions. Earthquakes occurred at depths between 1 and 7.6 km. The largest event was recorded at 0857 on 10 February; it was a local M 1.1, N of Arenas Crater at a depth of 5.7 km. Significant amounts of water vapor and gas rose from the crater during the week. A gas, steam, and ash plume rose 1.5 km and drifted NW on 13 February. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 26 January-1 February seismicity at Nevado del Ruiz was characterized by long-period earthquakes and continuous volcanic tremor associated with gas-and-ash emissions. Earthquakes occurred at depths between 1 and 8.7 km. The largest event was recorded at 1300 on 28 January; it was a local M 0.9, N of Arenas Crater at a depth of 3.2 km. Significant amounts of water vapor and gas rose from the crater during the week. A gas, steam, and ash plume rose 850 m and drifted NW on 26 January. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 22-28 December seismicity at Nevado del Ruiz was characterized by long-period earthquakes and continuous volcanic tremor associated with gas-and-ash emissions. Earthquakes occurred at depths between 1 and 7.7 km. The largest event was recorded at 1457 on 28 December; it was a local M 1.4, 7 km N of Arenas Crater at a depth of 5.4 km. Significant amounts of water vapor and gas rose from the crater during the week. A gas, steam, and ash plume rose 1.7 km and drifted NW on 28 December. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that a volcanic tremor signal at 0214 on 18 December may have been associated with an ash emission from Nevado del Ruiz, although dark conditions prevented visual confirmation. Volcanic tremor which began at 1247 on 20 December was associated with an ash emission observed by numerous people both near the volcano and in Manizales (30 km NW). Many also reported a strong sulfur odor. The permanent DOAS (Differential Optical Absorption Spectrometer) station detected the highest levels of sulfur dioxide measured since the current activity began. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 9-16 November seismicity at Nevado del Ruiz was characterized by long-period earthquakes and short-duration volcanic tremor associated with gas-and-ash emissions. Earthquakes occurred at depths between 1 and 8.1 km. The largest event was recorded at 1637 on 9 November; it was a local M 1.5, E of Arenas Crater at a depth of 2.1 km. Significant amounts of water vapor and gas rose from the crater during the week. A gas, steam, and ash plume rose 2 km and drifted NW and SW on 14 and 17 November. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 27 October-2 November seismicity at Nevado del Ruiz was characterized by long-period earthquakes and short-duration volcanic tremor associated with gas-and-ash emissions. Earthquakes occurred at depths between 1.3 and 8.2 km. The largest event was recorded at 1409 on 31 October; it was a local M 1.3, SW of Arenas Crater at a depth of 2.6 km. Significant amounts of water vapor and gas rose from the crater during the week. A gas, steam, and ash plume rose 1.7 km and drifted NW on 30 October. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 20-26 October seismicity at Nevado del Ruiz was characterized by long-period earthquakes and short-duration volcanic tremor associated with gas-and-ash emissions. Earthquakes occurred at depths between 1.1 and 8.8 km. The largest event was recorded at 0953 on 21 October, a local M 2.3, near Arenas Crater at a depth of 3.4 km. Significant amounts of water vapor and gas rose from the crater during the week. A gas, steam, and ash plume rose 2 km on 22 October. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 13-19 October seismicity at Nevado del Ruiz was characterized by long-period earthquakes and short-duration volcanic tremor associated with gas-and-ash emissions. Earthquakes occurred at depths between 0.9 and 8.5 km. The largest event was recorded at 2328 on 13 October, a local M 1.8, near Arenas Crater at a depth of 2.7 km. Significant amounts of water vapor and gas rose from the crater during the week. A gas, steam, and ash plume rose 1.8 km and drifted NW on 17 October. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 6-12 October seismicity at Nevado del Ruiz was characterized by long-period earthquakes and short-duration volcanic tremor associated with gas-and-ash emissions. Earthquakes occurred at depths between 1.4 and 8.2 km. The largest event was recorded at 0802 on 11 October, with a local M 3.4, near Arenas Crater at a depth of 3.6 km. That event was felt by residents and corresponded to a seismic increase at the NE part of Arenas Crater. Thermal anomalies over the crater were detected in satellite images during 7 and 9-10 October. Significant amounts of water vapor and gas rose from the crater during the week. A gas, steam, and ash plume rose 1.7 km and drifted NW on 8 October. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 28 September-5 October seismicity at Nevado del Ruiz was characterized by long-period earthquakes and short-duration volcanic tremor associated with gas-and-ash emissions. Earthquakes occurred at depths between 0.6 and 7 km. The largest event was recorded at 0138 on 4 October; the event was a local M 1.4, occurring at a depth of 4 km. Minor thermal anomalies over the crater were detected in satellite images on 28 and 30 September. Significant amounts of water-vapor and gas were emitted from the crater during the week. On 29 September a gas-and-ash plume rose 2 km above the crater and drifted mainly NW. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 22-28 September seismicity at Nevado del Ruiz was characterized by long-period earthquakes and short-duration volcanic tremor associated with gas-and-ash emissions. Earthquakes occurred at depths between 1.3 and 7.5 km. The largest event was recorded at 1027 on 22 September, a local M 1.1, near Arenas Crater at a depth of 4 km. Water-vapor, gas, and ash plumes rose 2 km above the crater and drifted mainly NW. Thermal anomalies over the crater were sometimes detected in satellite images; a significant anomaly was detected on 26 September. A pulse of tremor at 1633 on 28 September was accompanied by an ash emission that drifted E and W. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported that during 15-21 September seismicity at Nevado del Ruiz was characterized by long-period earthquakes and short-duration volcanic tremor associated with gas-and-ash emissions. Increases in seismicity were detected on 15 and 20 September, SW and N of Arenas Crater, respectively; the earthquakes were located at depths between 2.2 and 6.5 km, and were a maximum local M 2. Water-vapor-and-gas plumes rose 2.5 km above the crater and drifted mainly NW, and were sometimes tinged gray due to the presence of ash. Ashfall was reported in Manizales (30 km NW) and Pereira (40 km WSW). The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

On 14 September Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico y Sismológico de Manizales reported continuing volcanic tremor at Nevado del Ruiz associated with gas-and-ash emissions. At 0725 an ash emission was also recorded by the webcam. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico and Sismológico de Manizales reported that during 1-7 September seismicity at Nevado del Ruiz was characterized by long-period earthquakes and short-duration volcanic tremor associated with gas-and-ash emissions. In addition, volcano-tectonic (VT) events occurred at depths between 0.4 and 6.4 km. The largest VT event was recorded at 0017 on 2 September, was a local M 1.4, and was 1.5 km NE of Arenas Crater at a depth of 3.8 km. Incandescence was recorded by a webcam installed near the volcano. Water-vapor-and-gas plumes rose 1.4 km above the crater and drifted NW, and were sometimes tinged gray due to the presence of ash. Ashfall was reported in Manizales (30 km NW) and Pereira (40 km WSW). The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico and Sismológico de Manizales reported that during 25-31 August seismicity at Nevado del Ruiz increased; the network detected a large number of long-period earthquakes, and several episodes of tremor associated with gas-and-ash emissions. Water-vapor-and-gas plumes rose 2 km above the crater and were sometimes tinged gray due to the presence of ash. Volcano-tectonic (VT) events occurred at depths between 0.77 and 6.77 km. The largest VT event was recorded at 0144 on 27 August, was a local M 1.1, and was 2.8 km SW of Arenas Crater at a depth of 4.69 km. Periods of very-high-energy tremor were detected on 31 August. According to a news article, La Nubia airport ceased operations on 31 August due to ash emissions.

On 23 August Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico and Sismológico de Manizales reported that volcanic tremor associated with ash emissions from Nevado del Ruiz continued. Ashfall was confirmed by officials from Civil Aviation and the Parque Nacional Natural de los Nevados, as well as residents in Pereira. Earthquakes were located under the NE part of Arenas Crater, at depths between 3.5 and 5 km. The largest event was local M 2.

Based on satellite images, the Washington VAAC reported that on 6 August two small ash puffs from Nevado del Ruiz rose to an altitude of 7.3 km (24,000 ft) a.s.l. and drifted NW. Ash emissions later that day drifted over 20 km NW.

The Washington VAAC reported bright thermal anomalies at Nevado del Ruiz visible in satellite images on 31 July; a brief increase in seismicity was detected, and the thermal anomaly was weaker in subsequent images.

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico and Sismológico de Manizales reported increased tremor at Nevado del Ruiz during the morning of 26 July. The webcam recorded an ash plume at 0830 that rose 3 km and drifted NW to SW; ashfall was reported in areas downwind including the municipalities of Chinchiná, Palestina, Santa Rosa de Cabal, Dosquebradas, and Pereira. Later than day seismicity decreased and the webcam showed gas-and-steam emissions that rose 300 m.

According to the Washington VAAC, the Manizales Observatory reported that activity at Nevado del Ruiz increased at 0530 on 18 July, and an ash plume rose to an altitude of 6.1 km (20,000 ft) a.s.l. Weather clouds prevented satellite and webcam views. Later that day the Bogata MWO reported that an ash plume drifted SW at an altitude of 6.1 km (20,000 ft) a.s.l., and a NOTAM detailed an ash plume drifting S at the same altitude.

Based on webcam views and information from the Manizales Observatory, the Washington VAAC reported that on 6 June an ash emission from Nevado del Ruiz rose 800 m above the summit and drifted WNW. Weather clouds prevented satellite views.

The Washington VAAC reported that on 26 May an ash emission from Nevado del Ruiz was visible in the webcam and reported by the Air Force Weather Agency (AFWA). Weather clouds prevented satellite views.

Based on a SIGMET notice from the Bogota MWO, the Washington VAAC reported that on 18 and 19 December ash plumes from Nevado del Ruiz rose to altitudes of 7.9 and 9.1 km (26,000 and 30,000 ft) a.s.l., respectively. The plumes drifted SSW. A faint thermal anomaly was detected between cloud cover.

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico and Sismológico de Manizales reported that on 15 December at 0738 seismic signals indicating ash emissions were detected, and then confirmed, by a local webcam and residents. Based on a pilot observation, the Washington VAAC noted that an ash plume drifted almost 20 km S that same day. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico and Sismológico de Manizales reported that during 8-14 July gas-and-steam plumes rose 600 m above Nevado del Ruiz’s crater and drifted NW. Sulfur dioxide emissions were significant. Seismicity was dominated by volcano-tectonic signals. The earthquakes were located NE, S, and SW of Arenas Crater; the largest was a M (local) 2.1 located SW of Arenas Crater at a depth of 3.4 km. Shallow seismicity associated with fluid movement occurred S and SE of the crater. During 11-12 July continuous volcanic tremor associated with emissions was detected; ashfall was reported in Los Nevados Natural Park and in some parts of Manizales (30 km NW). The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

INGEOMINAS reported that during 13-14 April seismicity associated with fluid movement beneath Nevado del Ruiz was detected along with volcano-tectonic earthquakes. The earthquakes were located NW of Arenas Crater at depths between 5 and 9 km; the largest was a M 2.6, felt by officials of Los Nevados National Park in the area of Brisas (50 km SW). During the early morning of 14 April webcams recorded a gas-and-ash plume that rose 630 m and drifted NW. On 15 April a M 3 volcano-tectonic earthquake was located NW of Arenas Crater at a depth of 6.6 km. Later that day a M 2.5 volcano-tectonic earthquake was located again NW of Arenas Crater at a depth of 5.78 km. On 16 April at 0714 a M 3.2 earthquake was located in the same area at a depth of 6.22 km. Earthquakes continued to be felt by officials in the National Park. A gas-and-steam plume rose 1 km above the crater and drifted SW. Sulfur dioxide emissions were significant and deformation was detected. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

Based on analysis of satellite imagery, the Washington VAAC reported that on 16 January a diffuse plume from Nevado del Ruiz possibly contained ash and drifted almost 55 km NE. A thermal anomaly was also detected. INGEOMINAS reported significant emissions of mostly gas and steam during 14-20 January; plumes rose 1.6 km above the crater and drifted E and SE, and then W during the later part of the week.

Based on analysis of satellite imagery and reported seismic activity, the Washington VAAC issued a notice about a possible eruption from Nevado del Ruiz on 15 November. Cloud cover prevented observations of a possible ash plume but elevated seismicity was detected. A few hours later seismicity decreased and a faint thermal anomaly was detected. About six hours after that seismic activity remained low and no anomaly was detected.

According to INGEOMINAS, on 5 September the Observatorio Vulcanológico and Sismológico de Manizales reported that seismicity at Nevado del Ruiz significantly decreased, both in the number and magnitude of the earthquakes. Field measurements and analysis of satellite imagery continued to show a significant amount of sulfur dioxide in the atmosphere. A steam-and-gas plume rose 400 m and drifted W. Later that day, INGEOMINAS decreased the Alert Level to III (Yellow; "changes in the behavior of volcanic activity").

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that during 30-31 August variations in volcanic tremor amplitude were detected at Nevado del Ruiz, possibly associated with continuing gas and ash emissions. Seismic activity was low during 2-4 September. Cloud cover mostly prevented observations of the volcano; a white gas plume rose 200 m on 4 September and drifted W and SW. During 3-4 September field measurements and analysis of satellite imagery showed a significant amount of sulfur dioxide in the atmosphere. The Alert Level remained at II (Orange; "eruption likely within days or weeks").

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that during 22-28 August variations in volcanic tremor amplitude were detected at Nevado del Ruiz, possibly associated with continuing gas and ash emissions. Cameras located near the volcano showed gas plumes rising 800 m above the crater and drifting NE on 23 August, and rising 1 km the next day. Gas plumes rose 300-500 m and drifted W and N during 26-28 August. Field measurements and analysis of satellite imagery showed a significant amount of sulfur dioxide in the atmosphere during 24 and 26-27 August. The Alert Level remained at II (Orange; "eruption likely within days or weeks").

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that during 15-16 and 18-21 August variations in volcanic tremor amplitude were detected at Nevado del Ruiz, possibly associated with continuing gas and ash emissions. Earthquakes indicative of fracturing rock were located SE at depths of 1.5-4 km, with magnitudes less than 1. One event on 18 August was a M 2, located 1.17 km SW of Arenas Crater at a depth of 3.6 km. Web cameras showed gas-and-ash plumes rising 400 m and drifting W and NW during 15-16 August. A gas plume rose 400 m and drifted NW on 19 August. During 20-21 August field measurements and analysis of satellite imagery showed a significant amount of sulfur dioxide in the atmosphere. The Alert Level remained at II (Orange; "eruption likely within days or weeks").

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that during 10-13 August low levels of tremor were detected at Nevado del Ruiz, possibly associated with continuing gas and ash emissions. On 12 August a total of 140 low-magnitude earthquakes (M < 1.8) were detected in a seismic swarm that began at 0956 and ended at 1800. The earthquakes were located about 4 km WSW of Arenas Crater at depths of less than 5 km. A gas-and-ash plume observed with a web camera rose 1 km above the crater and drifted W. Ashfall was reported in Brisas (50 km SW). Satellite images showed continuing sulfur dioxide emissions. On 13 August a seismic swarm was characterized by events less than M 1, and located NE of Arenas Crater at depths between 3 and 5 km. A thin layer of ash was deposited at the observatory in Manizales. Weather conditions prevented observations of the volcano. The Alert Level remained at II (Orange; "eruption likely within days or weeks").

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that during 3-6 August low levels of tremor were detected at Nevado del Ruiz, possibly associated with continuing gas and ash emissions. Ashfall and a strong sulfur odor were reported in Manizales (30 km NW) and in the municipality of Chinchiná (30 km WNW). Satellite images showed continuing sulfur dioxide emissions on 6 August. Web cameras near the volcano recorded a gas-and-steam plume rising 700 m that drifted SW on 8 August. The Alert Level remained at II (Orange; "eruption likely within days or weeks").

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that on 31 July web cameras at Nevado del Ruiz showed gas-and-ash plumes rising 300 m above the crater. Seismicity was low. The Alert Level remained at II (Orange; "eruption likely within days or weeks").

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that during 22-24 July low levels of tremor were detected at Nevado del Ruiz, possibly associated with continuing gas and ash emissions. Satellite imagery and ground-based observations on 22 July showed high levels of sulfur dioxide emissions. The Alert Level remained at II (Orange; "eruption likely within days or weeks").

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that on 11 July seismicity at Nevado del Ruiz decreased, however the data continued to indicate gas and ash emissions during 11-17 July. Satellite imagery and ground-based observations showed sulfur dioxide emissions. On 11 July ashfall was reported in Pereira (40 km WSW). The Alert Level remained at II (Orange; "eruption likely within days or weeks").

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that during 5-9 July seismic signals at Nevado del Ruiz indicated continuing gas and ash emissions. On 6 July a gas-and-ash plume rose 400 m above the crater and drifted SW. On 8 July gas-and-ash plumes rose 400-500 m above the crater and drifted NW and W. The Washington Volcanic Ash Advisory Center (VAAC) reported that a gas-and-ash plume was observed in satellite imagery drifting 65 km. On 9 July INGEOMINAS noted that a gas-and-ash plume, observed with a web camera, rose around 400 m above the crater. On 10 July satellite imagery and ground-based observations showed sulfur dioxide emissions. The Alert Level remained at II (Orange; "eruption likely within days or weeks").

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that a significant concentration of sulfur dioxide was detected rising from Nevado del Ruiz during 28-29 June. Seismic signals indicated continuing gas and ash emissions.

On 30 June an eruption produced an ash plume that rose 8 km above the crater and drifted SW. The Alert Level was raised to I (Red; "imminent eruption or in progress"). Ashfall was reported in areas near the volcano including Manizales (30 km NW) and Villamaría (28 km NW). According to news reports, communities around the volcano evacuated, and airports in Manizales, Pereira, and Armenia closed. By 2 July seismicity had decreased to low levels; the Alert Level was lowered to II (Orange; "eruption likely within days or weeks"). The Washington VAAC reported that a 7.5-km-wide ash plume was detected in satellite imagery drifting 75 km W. INGEOMINAS noted that seismic signals indicated continuing gas and ash emissions on 3 July.

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that a high concentration of sulfur dioxide was detected during 20-22 June. Reports on 22 and 25 June stated that seismic signals had indicated continuing gas and ash emissions. Web camera images showed steam-and-gas plumes rising 300 m and drifting NW on both days. On 26 June seismicity was low. Sulfur dioxide emissions continued to be significant. The Alert Level remained at II (Orange; "eruption likely within days or weeks") on 26 June.

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported on 15 June that satellite image analyses and field observers of Nevado del Ruiz indicated significant sulfur dioxide emissions. Seismic signals on 15 and 18 June indicated continuing ash emissions. Based on analysis of satellite imagery and web camera views, the Washington VAAC reported that on 17 June a 5.5-km-wide gas plume, possibly containing ash, drifted more than 90 km NW. The VAAC noted on 18 June that INGEOMINAS reported a gas-and-ash plume drifting N and NW at an altitude of 6.1 km (20,000 ft) a.s.l. The plume was later detected in satellite imagery drifting more than 90 km NW. The Alert Level remained at II (Orange; "eruption likely within days or weeks") on 19 June.

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that a satellite image of Nevado del Ruiz acquired on 6 June showed an ash plume rising from the crater and drifting NW, and ash deposits on the N, NW, W and SW flanks. Based on analysis of satellite imagery and information from INGEOMINAS, the Washington VAAC reported that gas plumes possibly containing some ash drifted 75-110 km W, WNW, and N during 7 and 9-10 June. Ash plumes drifted almost 30 km SE on 8 June. INGEOMINAS reported that on 11 June seismic signals indicated continuing ash emissions. The Alert Level remained at II (Orange; "eruption likely within days or weeks").

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that on 4 June observations of Nevado del Ruiz during an overflight and through web cameras located in the vicinity of Arenas Crater revealed that gas-and-ash plumes rose at most 1 km above the crater and drifted W and NW. The next day seismic signals indicated that ash emissions continued. Field observers noted that sulfur dioxide emissions were significant. Based on analysis of satellite imagery, the Washington VAAC reported that an ash plume drifted 40 km W. The VAAC also noted that the Bogota MWO reported ash emissions drifting NNW at an altitude of 9.1 km (30,000 ft) a.s.l. The Alert Level remained at II (Orange; "eruption likely within days or weeks").

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that during 1-2 May both satellite images and field observers indicated that steam and sulfur dioxide emissions rose from Nevado del Ruiz. Seismicity continued to decrease. On 3 May the Alert Level was lowered to III (Yellow; "changes in the behavior of volcanic activity").

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that during 25-26 and 28-30 April video data and observers in the city of Manizales (25 km NW) indicated that gas-and-steam plumes rose 0.5-2 km above Nevado del Ruiz's crater. Cloud cover prevented observations on 27 April. Seismic signals indicated that an ash plume was emitted at 1248 on 30 April however cloud cover prevented observations. Seismicity fluctuated, but decreased. The Alert Level remained at II (Orange; "eruption likely within days or weeks").

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that during 18-19 April multiple seismic signals recorded at Nevado del Ruiz possibly indicated ash emissions, but they could not be verified due to weather conditions. Video data and observers in the city of Manizales (25 km NW) indicated that gas-and-steam plumes rose 900 m above the crater and drifted SE during the morning on 18 April. Satellite imagery detected a steam and sulfur dioxide plume the next day. During 20-24 April high levels of sulfur dioxide were detected in satellite images. Gas-and-steam plumes rose 1-2 km above the crater. The Alert Level remained at II (Orange; "eruption likely within days or weeks").

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that seismicity at Nevado del Ruiz decreased during 11-15 April then slightly increased during 16-17 April. Earthquakes were located below or N of Arenas crater at depths of 1.5-2 km during 11-12 April. Earthquake events at 1146 and 1149 on 15 April were possibly associated with ash emissions which were not verified due to weather conditions. Earthquakes detected on 16 April occurred E of Arenas crater at depths of 1.5-4 km.

Gas-and-steam plumes were observed mainly in satellite imagery, by cameras located near the volcano, and from the city of Manizales (25 km NW). On 12 April a sulfur odor was reported in the towns of Lebanon, Palocabildo, and Fresno (Tolima). Observes in Manila reported a gas-and-steam plume that rose 1.8 km above the crater on 16 April. The Alert Level remained at II (Orange; "eruption likely within days or weeks").

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that during 4-10 April seismicity at Nevado del Ruiz fluctuated but remained elevated. Sulfur dioxide emissions also remained high; occasionally a sulfur odor was reported in Manizales, about 25 km NW. On 5 April a volcano-tectonic earthquake, M 2.8, occurred below Arenas crater at a depth of 1 km, and was the highest magnitude earthquake recorded since February. During 4-6 April gas-and-steam plumes drifted NW. Gas-and-steam plumes rose 3 km above the crater on 7 April and 1.5 km above the crater on 10 April; the plumes drifted SE both days. The Alert Level remained at II (Orange; "eruption likely within days or weeks").

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that during 24-31 March seismicity at Nevado del Ruiz increased significantly. Earthquakes indicating rock fracturing occurred W of Arenas crater, exhibiting a pattern observed prior to the eruptions in 1985 and 1989, although less energetic. Starting at 1054 on 29 March, and lasting for about 25 minutes, more than 135 earthquakes were detected S of Arenas crater at a depth of 4 km. At 0400 on 31 March earthquake signals indicating rock fracturing and fluid movement markedly increased. The Alert Level was raised to II (Orange; "eruption likely within days or weeks"). Seismicity remained elevated during 1-3 April; earthquakes were located below Arenas crater at depths between 0.5 and 3 km.

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that during 19-25 March seismicity at Nevado del Ruiz increased and gas emissions remained at significant levels. Gas plumes rose 2 km above Arenas crater and sulfur dioxide odors were reported by local people. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

According to INGEOMINAS, the Observatorio Vulcanológico and Sismológico de Manizales reported that during 12-18 March seismicity at Nevado del Ruiz decreased but gas emissions remained at significant levels. Gas plumes rose 2 km above the crater and sulfur dioxide odors were reported by local people. The Alert Level remained at III (Yellow; "changes in the behavior of volcanic activity").

INGEOMINAS reported a significant increase in seismicity at Nevado del Ruiz during 5-11 March. On 8 March scientists aboard an overflight observed a gas plume that rose 1.4 km above Arenas crater, originating from multiple emission sources and thermally anomalous areas within the crater. They noted ash deposits on the glacier, near the crater rim and on the E flank, likely from an explosion on 22 February. Later that day a small explosion detected by the seismic network produced an ash emission that was observed with a camera installed in La Piranha (NW). Increased sulfur dioxide emissions were also detected. Fieldwork revealed ash deposits at the headwaters of Gualí River, SW of Arenas crater.

INGEOMINAS reported a gradual increase in seismicity from Nevado del Ruiz on 30 September. Earthquakes were located beneath the Arenas crater at depths of 0.5-2 km. The largest earthquake was M 1.9. A plume of white gas rose 700 m above the caldera and a sulfur odor around the volcano was reported. The Alert Level was raised to III (Yellow; "changes in the behavior of volcanic activity") on 1 October. The report also noted changes in deformation and geochemistry during the previous few months. Seismic levels fluctuated during 2-3 October.

The Washington VAAC reported that a plume at an altitude of 7 km (23,000 ft) a.s.l. from a possible eruption from Nevado del Ruiz on 17 July was stated in a Bogota MWO SIGMET notice. A second VAAC report less than an hour later noted a brief seismic signal alert had been issued and that meteorological cloud cover prevented observations of the volcano. Ash was not seen in satellite imagery later that day nor were there any additional reports of activity. [INGEOMINAS later confirmed that no eruption had occurred.]

On 9 June beginning at 2300 a swarm of volcano-tectonic earthquakes was recorded at Nevado del Ruiz. Next, hundreds of hybrid earthquakes were recorded, with more than 1,300 earthquakes occurring in 16 hours. High seismicity persisted for the following 3 days, with ~2,300 earthquakes recorded. This was the highest daily number of events recorded at Nevado del Ruiz since 1985. At the height of the activity the Alert Level was at Orange. In addition to heightened seismicity felt by residents near the volcano, jet-like sounds were reportedly heard that corresponded with some of the hybrid earthquakes, and the strong scent of SO2 was reported near the summit. No ash emissions were reported. By 13 June seismicity had decreased.

Information is preliminary and subject to change. All times are local (unless otherwise noted)

"Since late November 1984, local earthquakes have been felt near the summit. On 22 December, stronger earthquakes were detected, followed by a half hour of apparent harmonic tremor. During a visit to the crater in early January 1985, increased fumarolic activity, evidence of phreatic explosions, and the wide deposition of sulfur salts over the adjacent snowcap were noted. At times, a thin layer of ash had been ejected, which was analyzed by J. Tomblin and found to consist of alteration products and sulfur.

"Seismic activity continued, with 17 felt earthquakes in March and 18 in April. There are no operating seismographs in the region. Abnormal fumarolic activity also continued. The one hot spring with frequent temperature monitoring, NW of the crater, had not shown any variation in temperature.

Increased thermal and seismic activity have continued. Scientists from the Central Hidroeléctrica de Caldas visited the crater on 8 July and found evidence of increased activity since their previous visit on 22 February. They reported intense noise from the fumaroles as well as increased fuming which frequently made breathing difficult. Sulfur deposits were more extensive than those noted on their previous visit. On 22 February, there had been only a thin film of sulfur covering the surface near the fumaroles, but by July sand-sized material on the inner slopes of the crater was impregnated with sulfur deposits, creating a crust 10 cm thick. A new crack was observed near the crater rim; it was 1.5 m in length, 8 cm wide, and emitted hot gas and vapors. Ground temperatures were measured at various locations as the team descended into the crater. Isotopic studies are being conducted on waters collected at thermal vents on the flanks of the volcano.

Much of the bottom of the crater was covered by a green lake that emitted hot steam from its surface. On 22 February, the lake had a pH of 0.2. The water in the lake had risen at least 1 m since February, covering a mud pool and its surrounding "mud volcano" seen on 22 February, and also nearby fumaroles. Projecting into the SE side of the lake on 8 July was a peninsula that was thought to be the remnant of these features. An unusual thaw of the glacial ice that covers much of the summit area seemed to have contributed to the rise in the water level. Further evidence for thawing came from the presence of large blocks of ice scattered near the base of the crater.

A major increase in fumarolic activity producing an enormous yellow vapor cloud was reported on 23 July.

Ash emission began 11 September at about 1300 and lasted up to 7 hours, accompanied by a persistent roaring noise and electrical discharges. A few millimeters of ash fell on the cities of Manizales and Chinchiná (about 30 km NW and WNW of the volcano) and ashfall was reported to have locally reached 2 cm. A mudslide blocked a road on the E flank of the volcano. Before the eruption, tremors were recorded at about 1.5-hour intervals by seismographs about 3 km N, S, E, and W of the volcano's summit. Tremor was continuous during the ashfall, punctuated by discrete earthquakes.

On 13 September, morphological changes in the glaciers on the W side of Ruiz were reported by ground and air observers. New fissures were also reported on the icecap N of and below the active crater near a small growing pond on the glacier surface.

Nevado del Ruiz is a broad shield-shaped volcano covering more than 200 km2, composed of an extensive sequence of hypersthene-augite andesite, hornblende andesite, basaltic andesite, and dacite. Its 1595 eruption produced ash and lapilli falls and lahars that spilled down the Río Gualí and Río Lagunillas Valleys N and E of the volcano. Eruptions were reported in 1828 and 1829, but no descriptions exist. Apparently these events produced no tephra. Ruiz is also reported to have been "smoking" in 1831 and 1833 (Herd, 1982).

After a series of magnitude-4 earthquakes on 22 December marked the onset of stronger seismicity, 25-30 events were felt each month until an episode of strong phreatic activity on 11 September. Seismologists have located about 100 earthquakes, centered 1-2 km NW of the active crater at average depths of 0-2 km below sea level. Increased thermal and phreatic activity began in early 1985 and continued after the 11 September episode into early October. No juvenile material has been recognized in the 1985 tephra.

The vigorous 11 September phreatic activity began at 1330 from the summit crater (Arenas) and had ended by the next morning. Ashfall was 1 cm or less near the volcano and a trace of ash reached Manizales and Chinchiná. Lithic blocks were deposited on snowfields as much as 2 km from the crater. Thunderous detonations and summit-area lightning accompanied the activity. A small- to moderate-sized lahar began at 1830, advancing 27 km down the valley of the NE flank's Río Azufrado, from 4,700 m to 3,000 m altitude. As it travelled at an estimated 10-30 km per hour, the lahar left the river channel at various locations, particularly along curves, and rose as high as 10-20 m up canyon walls. Valley residents were placed on alert but have not been evacuated. A hazard map has been prepared by the international team studying the volcano and distributed to Red Cross and Civil Defense officials.

For 5 days preceding the 11 September activity, seismographs registered a very regular pattern consisting of 15 minutes of strong high-frequency tremor every hour. Although similar seismicity was recorded during three other periods of a day or less in August and September, none of these episodes was as intense or long-lasting.

Phreatic activity was continuous for the rest of September, emitting variable amounts of ash, typically darkening the snow to a few kilometers from the crater. Heavier emissions occurred on 23, 24, and 29 September, producing dense dark plumes that deposited trace ashfalls more than 10 km from the crater and lithic blocks on nearby snowfields. Activity had declined by early October. A steam plume 1-3 km high was visible daily but contained no obvious ash. Seismographs recorded 5-10 microseismic events per day and an irregular harmonic tremor that may have been related to the steam plume. Geologists visited the crater at the beginning of October, noting a slight decline in fumarolic activity and little ash emission. Glaciers seemed unchanged since September.

An explosive eruption on 13 November melted ice and snow in the summit area, generating lahars that flowed tens of kilometers down flank river valleys, killing more than [25,000] people. This is history's fourth largest single-eruption death toll, behind only Tambora in 1815 (92,000), Krakatau in 1883 (36,000) and Mt. Pelée in May 1902 (28,000). The following briefly summarizes the very preliminary and inevitably conflicting information that had been received by press time. [An 11-page Appendix that originally appeared at the end of this issue presented a more detailed but equally preliminary chronology.]

After the moderate explosive activity (and 27-km lahar) of 11 September, there were several smaller ash ejection episodes through the end of September. Activity declined by early October to emission of a 1-3 km steam plume that contained no obvious ash. Low-level steaming with very little ash continued into late October, accompanied by occasional earthquakes, apparently tectonic, at about 12 km depth. Shallow (0-5 km) earthquakes, as many as 5 per day, were centered about 0.5 km N of the active crater. Three dry-tilt stations were established in late October, N, S, and W of the summit, and 11 days of data (before 5 November) suggested that some deflation was occurring.

Seismicity began to increase by 7 November, characterized by a series of high-frequency seismic swarms, although fewer than were associated with the 11 September ash emission. Continuous volcanic tremor began 10 November, but was weaker than the 11 September tremor.

No additional changes in seismicity were apparent before the onset of eruptive activity at about 1545 on 13 November. Ashfall reportedly began about 1600 in heavy rain at Mariquita (59 km NE of the summit) and lapilli with ash started falling about 1730 in Armero (46 km to the ENE) (figure 1). At 2109, Bernardo Salazar, tending seismic equipment 9 km from the summit, heard the start of strong explosions, much louder than those on 11 September, that shook the building and lit up the rain clouds "like a lamp." Heavy rains prevented Salazar from seeing the eruption column, but pumice to 15 cm in diameter began to fall by 2137, and 1-cm lapilli fell 18 km [W of] the summit. Ashfall at Armero increased about 2200. At 2220 a Caribbean Air Lines cargo plane flew through the eruption cloud at 8 km altitude. Pilot Manuel Cervera reported that "smoke" and the smell of sulfur filled the cabin, and his windows were etched. Two attempts to land at Bogotá failed, but he landed safely at Cali by putting his head out a side window.

Lahars traveled down 11 flank valleys, the most destructive inundating the city of Armero, where an estimated [21,000] of 25,000 residents died. The first mud, which reached Armero about [2335], was cold [reports that the mud became increasingly hot were incorrect]. Resident E. Nieto described "A frightening noise and then a blast of wind hit us and we saw fire falling from the sky." On the volcano's W flank, low-lying neighborhoods of Chinchiná were also buried by mud, and officials estimated that [1,000] died there. Geologists estimated that the lahars advanced at 30-35 km per hour; Armero, at about 300 m elevation, is 5 vertical kilometers below the summit of Ruiz.

No pyroclastic flows were observed during the eruption, but geologists noted deposits with cross-stratification typical of those from surges. The Refugio (about 2 km NW of the summit) was knocked down and its walls scattered radially downslope.

In hand specimen, the 13 November tephra appeared to be a dacitic andesite, about 30-40% phenocrysts, with plagioclase dominant, more hornblende than pyroxene, and a trace of biotite. No estimates of tephra thickness, volume, or extent were available at press time, but ashfall was reported as far away as Táchira, Venezuela, 500 km to the NE. Tephra fell on all but the S and SE flanks.

NASA's Nimbus-7 polar orbiting satellite passed over Ruiz on 14 November at 1151. Using data from its TOMS instrument, Arlin Krueger calculated that a cloud containing more than 0.5 x 106 metric tons of SO2 covered a 650,000 km2 zone extending NE and slightly SW of the volcano (figure 2). Bernardo Londoño reported that the cloud extended to 10.5 km above the crater, but it remains uncertain whether the cloud reached the stratosphere. Heavy weather clouds prevented observation of the eruption on NOAA's weather satellite images. Lack of nearby wind data prevented precise altitude determination by correlation with cloud movement as indicated by Nimbus-7, but the available data suggested that it was upper tropospheric.

Figure 2. Preliminary SO2 data from the TOMS instrument of the Nimbus-7 satellite at 1151 on 14 November 1985. Each number or letter represents the average SO2 value within an area 50 km across. Values above 9 are represented by A, B, C, etc. The area of perturbed values is shaded. Courtesy of Arlin Krueger.

As of 20 November, no additional strong explosions had occurred. Geologists who flew over the volcano reported that the crater had enlarged to about 300 m in diameter and 200 m deep, and that new fumaroles had developed 500-900 m from the crater, but they observed no lava flow. Newly installed telemetering seismometers recorded harmonic tremor of varying amplitude beginning 18 November at 0200 that preceded ash emission at 0600. An episode of stronger tremor that lasted from about 0600 to 0945 the next day was followed by a small explosion at 1030.

Seismic swarms, latest with inflation; more on 13 November activity and products

Since the 13 November eruption, activity at Ruiz has been limited to emission of a vapor plume and a few seismic swarms, one accompanied by measureable inflation. Work by numerous geologists has yielded new information on the 13 November eruption, its products, and pre-eruption activity.

Pre-13 November activity. The most vigorous seismic energy release at Ruiz occurred in the days preceding the 11 September ash emission. The rate of energy release increased prior to the 13 November eruption, but more gradually than before the 11 September activity. Hypocenters were concentrated N and NE of the summit with best-located events concentrated at depths 0-1 km below sea level (figure 3).

The quoted material below is from a report from Rodolfo Van der Laat, Eduardo Parra, and Heyley Vergara.

"After 11 September, when there was a significant ash emission, activity at Ruiz had decreased notably through 10 November. The activity caused concern in Manizales (30 km NW of Arenas Crater), but the presentation by INGEOMINAS of a preliminary volcanic risk map (figure 4) calmed the population.

Figure 4. Volcanic risk map presented by geologists from INGEOMINAS and the Univ. de Caldas to officials and the press before the 13 November 1985 eruption. Some redrafting has been done to facilitate reproduction in the Bulletin, but boundaries of hazard zones are unchanged.

"Seismic activity reached a maximum of 60 events per day 19-21 October, declining by 3 November to 3-5 daily locatable events. An increase in temperature of the thermal vent 'La Hedionda' (on the NE flank) may have been related to the increase in seismic activity.

"The height of the plume during this period decreased from about 3 km at the end of September and the beginning of October to about 800 m, with an occasional nucleus of ash 200-300 m high. There were two main fumarolic vents: one yellowish (sulfur), the smaller one gray/coffee-colored (ash derived from mud).

"A tilt network was established, detecting a general deflation 26 October-3 November, with small pulses of inflation of the order of 5-10 µrad per day. At the beginning of November, the first measurement was made of a geodesic net to monitor horizontal deformation by the Instituto Geográfico Agustín Codazzi."

13 November eruption and products. Details of the 13 November eruption sequence remain uncertain and field investigations were still in progress at press time. An initial explosion at 1530 deposited a very thin, fine-grained layer of ash around the summit and NNE of the volcano. The main explosion started at 2108 or 2109 and continued for 20-30 minutes. Five kilometers from the crater, tephra from the main explosion was 7 cm thick and included 30-cm pumice fragments, but the deposit thinned rapidly and was only 1-2 mm thick at Armero with similar amounts at Mariquita and Honda (75 km NE). Preliminary estimates by Haraldur Sigurdsson and Steven Carey place the volume of tephra at roughly 39 x 106 m3. Cloudy weather and lack of nearby wind data on 13 November impeded determination of the height of the Ruiz eruption column. Based on the position of tephra diameter isopleths, Sigurdsson and Carey inferred that the top of the eruption cloud reached [31] km altitude, but emphasized that most of the tephra probably remained in the upper troposphere [Naranjo and others, 1986].

Mudflows that moved E down the valleys of the Lagunillas and Azufrado rivers and inundated Armero were overlain by airfall tephra within 5-10 km of the volcano. However, the mudflow that moved W down the Río Claro valley to Chinchiná contained fresh pumice, and the fluid mudflow that traveled down the Gualí river washed tephra off vegetation, suggesting that both were generated after tephra ejection. The Armero mudflows emerged from both the Lagunillas and Azufrado valleys, which join upstream from the city. The first wave of mud, probably from the Lagunillas, was apparently colder, lighter colored, more water-rich, and formed a more extensive deposit than the second wave, probably from the Azufrado, which was hotter, coarser, and darker-colored. Donald Lowe estimated that outflow from the mouth of the Río Lagunillas reached about 47,500 m3/s. Preliminary calculations by Sigurdsson and Carey yield a volume of about 30-60 x 106 m3 for the deposits of the Armero, and Gualí and Chinchiná valley mudflows, plus about 30-90 x 106 m3 of water, roughly 6-18% of the pre-eruption volume of the summit ice cap. The Lagunillas mudflow probably included water from a lake that had been trapped behind a debris dam in that valley's headwaters for at least several months. Other estimates suggested that about 5% of the summit ice was removed during the 13 November eruption.

Preliminary chemical analyses of a few samples of the 13 November pumice suggest that it is a hypersthene andesite, very similar to an earlier pumice that was probably from Ruiz's last large eruption, in 1595. Little systematic variation was found in different-colored samples that had suggested mixed magma in hand specimen (table 1).

Post-13 November activity. No significant eruptive activity occurred in the succeeding weeks. The vapor column varied in height from 200-300 m to 1-1.4 km. Rates of SO2 emission measured by COSPEC were 200 t/d on 18 November, 50 t/d on the 19th, and several thousand tons per day on 22 November. Possible new fissures have been observed near the summit along with possible development of a depression SW of the summit. However, the fissures may have been pre-existing features exposed by clearer weather and seasonal snowmelt. Slight advances of some of the summit glaciers have been noted, but no large-scale ice movements were apparent and there was no evidence of significant melting from below.

Six telemetering seismometers have been installed, ringing the summit at elevations of 4,000-4,500 m, supplementing the four-station seismic net that was in place before 13 November. Telemetering tiltmeters were emplaced at 4,200 m elevation on the NW flank, 4,600 m elevation on the NNW flank, and on the NE flank, and 8-10 EDM lines have been established, in addition to the dry-tilt network installed on the N flank in October.

Seismic energy release was at relatively low levels shortly after the 13 November eruption, but the slope of the energy release curve steepened in the succeeding weeks. Earthquake swarms that were small but of increasing energy occurred 19-20 and 27 November, and 6-7 December. Maximum magnitudes were 2.5-3 in the November swarms; the 6-7 December activity included two magnitude 3-3.5 shocks. Locations were available for only a few events, which were centered along a generally N-S trend, usually somewhat N of the crater. The swarms were not accompanied by measurable tilt episodes or obvious changes to the plume. The rate of seismic energy release doubled during the first day of a stronger swarm 12-13 December and Civil Defense personnel were put on alert. The same day, the NW flank tiltmeter recorded a 5 µrad tilt event, the first change recorded in the weeks since it was installed, and a NW flank EDM line shortened 14 cm between measurements 11 and 13 December. However, seismicity declined 13 December, and the seismic energy release curve was nearly flat 14-17 December.

Explosive activity on 4 January ejected a small amount of ash and was accompanied by vigorous seismicity. The 4 January activity did not generate mudflows or cause any apparent changes in river flow, but residents of low-lying areas were temporarily evacuated as a precautionary measure.

A series of earthquake swarms followed the 13 November eruption, including a strong swarm 12-13 December that was accompanied by deformation (figure 5). Seismicity then declined briefly, followed by a period of stronger seismicity 22-24 December, then diminished again at the end of December to about 10 events per day (M >= 0) and brief bursts of tremor. Epicenters were generally S of the active crater, extending E and W under the flanks. Before the 4 January eruption, focal depths decreased from 4-8 km to 2-5 km (below a datum at 3.8 km above sea level). EDM lines on the SW, N, NE, and E flanks began to show changes in the rate and/or direction of deformation between 19 and 24 December. Equipment problems prevented remeasurement of EDM lines immediately before the 4 January eruption, so the amount of pre-eruption inflation is uncertain. The net change in the lengths of several radial lines (of 5 km average length) measured 3 days after the eruption was about 10 cm, but this figure probably included substantial post-eruption deflation. By 28-30 December, small but distinct changes in rate or direction of tilt had begun to appear on all four electronic tilt stations (at 4,100 m elevation on the NE flank, 4,800 m on the W flank, about 3,900 m on the NW flank, and 4,600 m on the SE flank).

Figure 5. Seismic energy release at Ruiz volcano, 20 July-19 December 1985. Timing of eruptions on 11 September and 13 November are shown. Courtesy of the Comité de Estudios Vulcanológicos.

Movement of cracks in summit glaciers continued through December and early January at roughly constant rates. Extensional changes of 5-10 cm per day were measured near the head of the Azufrado valley, and both extensional and compressional motion of a few mm to 5 cm per day elsewhere. Little baseline data exist on typical rates of glacier advance on Ruiz.

Strong seismicity began 3 January at about 2320, and was saturating seismographs within less than an hour. The seismicity was initially characterized by superimposed high- and low-frequency tremor, but tremor amplitude declined somewhat around 0115 and low-frequency (2-2.5 Hz) tremor began to dominate the seismic records at 0128. B-type earthquakes and explosion events accompanied the tremor. Darkness initially prevented direct observations of the summit, but ash began falling about 0300. The eruption cloud was small, generally 300-600 m high, occasionally rising to 1 km above the summit. Ashfalls were minor, concentrated around the summit and in a narrow zone to the WNW. Several hundred meters from the vent, new ash was only about 7 mm thick; 3 km downwind the deposit was only 2 mm deep; and only traces of ash were found more than 10 km away. Vigorous seismicity continued until about noon, then declined slowly until the eruption ended in mid-afternoon.

Evacuations of about 15,000 people from low-lying areas of the valleys of the Azufrado, Lagunillas, Recio, Gualí, Sabandija, and Chinchiná rivers began 4 January at about 0600. Most residents returned to their homes shortly after the eruption, but about 2,000 people remained evacuated 10 days later.

Smaller earthquake swarms occurred 5-7 January, then seismicity declined to about 1-2 A- or B-type events per hour, generally with magnitudes of 0 or less. No additional explosions or major increases in seismicity had occurred as of mid-January.

Seismicity and deformation continued through early February, but there had been no eruptive episodes since the minor explosive activity on 4 January. Plume heights varied from 100 to ~1,000 m above the summit (often in a single day), partly dependent on the amount of rainwater accumulated in the crater. Weak ash emission occurred on some days, falling near the crater.

After the 4 January ash emission, about four mixed-frequency seismic events were recorded/day, declining to <1/day since 1 February. Between mid-January and mid-February, there were normally about 10 high-frequency events/day, with two small crises on 8 and 14 February of about 20 events each (M < 1). The low-frequency seismicity also registered about 10 events daily without significant variations. Depths were between 2 and 6 km below a datum at 3.8 km above sea level, with epicenters typically <2 km from the crater, closer than in December. In early February, events were concentrated in the NE part of the volcano [see also 11:02].

An electronic tilt station W of the crater measured inflation values of the order of 0.5 µrad/day until the end of January, then dropped to 0.2-0.3/day, while deformation on other flanks remained at lower rates (<0.1 µrad). A deformation event that started 18 January was measured by continuously recording tiltmeters 2 and 5 km NW of the summit and by EDM stations radiating W from the summit. About 20-25 mm/km of inflation accumulated before the episode ended 4-5 days later. Another small inflation episode was in progress on 13 February, totaling ~8 mm/km at a reflector ~4 km NE of the summit.

By February, rates of fissure displacements in the summit glaciers had slowed by as much as an order of magnitude, to millimeters or occasionally centimeters/day. Increased melting was observed but attributed to sunnier weather. Melting of ice caused buckling of overlying tephra layers that were as much as 6 m thick near the summit, producing features reminiscent of tumuli that form on top of block lava flows.

An improved alert system and additional hazard study has permitted some of the persons that had remained evacuated since 4 January to return to their homes.

Colombian geologists reported that microseismic activity diminished between mid-February and mid-March, with an average of 5 high-frequency and 12-15 low-frequency events daily. Seismic events of mixed frequency also diminished, to 1/day. Activity on 28 February was abnormally low, with only one low-frequency event. Epicenters were within 3-4 km of the crater, and focal depths were at 1-3 km below a datum at 4.7 km above sea level . . . .

The vapor column reached heights of 100-800 m, without significant ash emission. Rates of SO2 emission measured by COSPEC in early March were generally of the order of 500 t/d; during the afternoon of 4 March, values of ~1,000 t/d were recorded, about the same as a month earlier. Fissures in pyroclastic material on glaciers on the NE part of the volcano showed displacements of 20-30 mm/day. Electronic tilt instruments recorded ~0.1 µrad of inflation/day on the W flank. No significant changes were detected on the E flank.

Additional seismic information comes from Jim Zollweg. "After the 13 November eruption, a 6-station telemetry network was installed around the volcano. All stations use vertical 1-Hz seismometers and radio-telemeter to the Comité de Estudios Vulcanológicos headquarters in Manizales. Sites were chosen 4-10 km from Arenas, Ruiz's active crater. The first station was operating on 17 November, four stations were functioning by 28 November, and all six by 2 December, although some locations were changed later. Recording is analog using pen-and-ink or smoked paper.

"Seismic activity was moderate in November and December, consisting chiefly of high-frequency earthquakes with magnitudes to 3.5. There were about half as many low-frequency as high-frequency earthquakes during those months. About 1,300 earthquakes were counted in December (countable events had coda lengths of at least 5 seconds on the Olleta telemetry station). In January, activity was considerably lower because of a major decrease in the rate of occurrence of high-frequency events. Only a few surface-type events (gas emission signals or avalanches) were recorded by telemetry stations in any month. Instances of harmonic tremor sustained for more than a minute were also uncommon, particularly in December. The most important tremor recorded since the stations were installed occurred 3-5 January, preceding and during a minor eruption 4-5 January. This tremor was unusual because of its wide range of frequency content; frequencies of 7 Hz or more were recorded within a few hours of its onset, and frequencies as low as 0.7 Hz occurred later in the episode.

"Very few low-frequency earthquakes have been locatable, and those with epicenters that could be computed usually occurred near Arenas Crater (within the limits of accuracy of the solutions). More than 150 high-frequency earthquakes have been located, and 81 of the better solutions are plotted in figure 6. Events were located using the program HYPOINVERSE, and an ad-hoc crustal model based on geological considerations. Events plotted within the network have epicentral 95% confidence limits of 1 km or less. Those plotted outside the network may have epicentral 95% confidence limits as poor as 3 km, depending on distance from the network. Focal depths of the high-frequency events are usually between 2 and 5 km (beneath a datum at 4.7 km asl), but depend on the crustal model used. The probable error range makes it difficult to say whether there are significant depth differences between events.

Figure 6. Earthquake epicenters at Ruiz, 28 November 1985-21 January 1986. The 5,000-m contour is outlined. Filled triangles are telemetered stations; others are shown by open triangles. Not all were operating simultaneously; PIRT and ARBO are no longer operating. Courtesy of Jim Zollweg.

"Most of the high-frequency earthquakes occurred in one of two linear zones that intersect under the center of Ruiz. The E-W-striking zone is ~6 km long and was responsible for most of the seismicity. There is an interesting temporal pattern to the epicenters. Locations between 28 November and 5 December mainly fell in the central and eastern parts of the E-W zone, whereas vigorous swarms 6-13 December were mainly confined to the W half of the zone. There was a pronounced hiatus in high-frequency activity 14-21 December, followed by a swarm of events 22-25 December along the second zone, striking NW-SE. Between 26 December and 3 January, earthquakes reverted to a small area near the intersection of the two zones. High-frequency activity was comparatively low in the weeks following the 4-5 January eruption. First motions have been mostly compressions for nearly all events under Ruiz, suggesting a normal faulting environment. The data for the high-frequency sequences suggest the intrusion of dike-like bodies of magma along pre-existing fault zones."

Deformation monitoring has been summarized by Barry Voight. "Preliminary data from EDM reflector stations suggest that much of the observed summit surface deformation is due to ice movement decoupled from underlying bedrock. The data place constraints on the volume of rock susceptible to massive gravitational failure. EDM reflector stations (Martica and Finger) were installed at ~5,100 m elevation near the center of the Río Azufrado headwall. They were first measured on 14 February. Preliminary data from repeated distance measurements to 7 March suggest that the motions of glacier ice and underlying bedrock are decoupled. The fracture pattern observed on the surface of the summit plateau E of Arenas Crater dominantly reflects the propagation of crevasse patterns through the veneer of surficial deposits. The steady outward motion of the summit area as deduced from observation of surficial fracturing and EDM monitoring mainly reflects movements in glacier ice rather than motion of underlying bedrock. At other locations, some cracks give the appearance of possibly extending into bedrock, for example at the septum separating the crater from the headwall of the Río Azufrado. Although my current impression is that all of the headwall is probably stable with respect to deep-seated rock failure, the potential for mass movement involving a portion of the crater remains to be thoroughly evaluated."

Fabian Hoyos P. reported that isotopic analyses of several hot springs and the crater fumarole were sampled on 8 July 1985 in cooperation with the Central Hidroeléctrica de Caldas (CHEC). Isotopic compositions of 8 July 1985 hot spring samples were similar to those reported in 1968 and 1980 to CHEC. Water vapor collected the same day from the summit crater fumarole had an isotopic composition corresponding to that of meteoric water precipitated at ~2,800 m asl (table 2). Other samples collected until just before the 13 November eruption are being analyzed.

Between mid-March and mid-April, the height of the vapor column varied between 300 and 1,000 m, with SO2 content, measured by COSPEC, of 300-1,000 t/d. Rates measured in early March were generally of the order of 500 t/d, down from ~1,000 t/d a month earlier. Ash contents of the plume were low.

Seismicity during the period generally remained similar to the previous month. The number of high-frequency events declined slightly to ~3/day, while low-frequency shocks increased somewhat to an average of 12 daily. Depths of high-frequency events were as much as 8 km (below a datum at 4.7 km altitude), with ~65% of the events deeper than 3 km. Epicenters were dominantly in the S part of the volcano. Around 21 March there was a small seismic crisis with 24 low-frequency and 13 high-frequency events that were associated with a small ash emission. On 6 and 7 April, two significant low-frequency events were registered, but they were not accompanied by any other activity. Colombian geologists noted that regional earthquakes could have some influence on the increase in low-frequency seismicity.

During the second week in April, data from EDM lines revealed slight deflation (of the order of 1 mm/day) at stations on the N and E sectors of the volcano. In contrast, a station in the SW sector (CISNE) indicated an inflation of ~3 mm daily during the same period. Dry and electronic tilt measurements have not shown significant changes. Rates of movement of ice near the summit have slowed since January to less than half the December rates.

Activity during 15 April-15 May. A substantial increase in seismic activity began in late April and was continuing in mid-May. Seismic records suggested that a minor ash emission occurred on 4 May, and ashfalls were reported within 3 km of the summit.

Seismicity remained at moderate levels until 20 April, when there was a swarm of 37 high-frequency events, with maximum magnitudes of 2.0. On 24 April a swarm of 24 low-frequency events was apparently accompanied by an increase in the SO2 content of the vapor column that emerges from Arenas Crater. No other eruptive changes were reported.

Harmonic tremor began on 4 May at 1300. A seismic signal with a maximum amplitude of 8 mm started at 1445 and lasted ~4 minutes, apparently marking a brief ash emission that was not directly observed. Periods of tremor of variable duration were recorded on 5, 7, 8, 9, 10, and 11 May, the longest ~26 hours, separated by brief quiet intervals. Tremor frequency remained relatively constant (5-6 Hz) but amplitudes varied significantly, reaching a maximum of 20 mm on 4 May between 2243 and 2300. The beginnings and ends of tremor episodes were often gradual, but on some occasions tremor ceased suddenly. On 7 May between 0000 and 0200, a swarm of high-frequency earthquakes was detected. One event had a magnitude of more than 2.0, the largest registered at Ruiz since December 1985.

dry-tilt data showed inflation of between 2 and 15 µrad on three of the four stations, while the fourth, on the NW flank, indicated 37 µrad of deflation. Electronic tilt measurements and EDM lines showed only minor changes.

COSPEC data yielded rates of SO2 emission that varied between 24 and 5,000 t/d. However, a station that measures wind direction and intensity has failed, and the raw COSPEC data can no longer be corrected for those factors.

Activity during 15 May-15 June. Seismic activity . . . remained dominated by harmonic tremor since 4 May. The tremor had a dominant frequency of 5 Hz, but strong components of frequencies as low as 1 Hz have been recorded at times. During a period of weak tremor on 29 May, there was a several-hour swarm of low-frequency events. About 1 day before this swarm, electronic tiltmeters began to show fluctuations, with daily changes of 1.5 µrad and periods of several hours to a few days. The pattern of all deformation measurements (EDM, dry-tilt, electronic tilt, and levelling) was inflationary as of mid-June, although small and irregular (0.4 µrad/day).

After 6 weeks, harmonic tremor fell to the noise level on 14 June and the number of low-frequency events increased again. Five shallow events, with durations of up to 3 minutes, were counted. Poor visibility prevented correlations with crater activity. Tremor resumed during the evening of 16 June and was continuing the next day.

Ash emissions were evident on many days, sometimes depositing thin layers on fresh snow. When winds forced the column down, a sulfur smell was strong as far as several tens of kilometers from the crater. During most of the few overflights permitted by weather conditions, the crater fumaroles again showed distinct behavior, with one emitting ash, the other only vapor. The most recent COSPEC data, collected at the beginning of June, showed an SO2 emission rate of 3,500-5,200 t/d, in contrast to previous weeks in which values fluctuated between 250 and 1,200 t/d. No significant changes to the summit ice cap were observed.

Because of the significant increase in activity, on 12 June the government released orders for permanent evacuation of a zone within 10 km of Arenas Crater (population ~1,700), and strongly recommended that the area within a radius of 10-20 km also be evacuated (population 8,000-10,000).

Activity during 15 June-10 July. "Harmonic tremor, which started on 4 May, still dominated the pattern of seismic activity at Ruiz as of 10 July. Several times, swarm-like single event sequences of mostly low-frequency earthquakes changed the pattern for hours or even days.

"On 5 July the tremor declined almost to background level. At the same time the frequency of events increased sharply, totaling ~300 (minimum 5-second duration) by 10 July at Olleta station (4.2 km W of Arenas Crater), the highest number for a 5-day period since the 13 November eruption. The maximum magnitude was 2.2 and most events were of the low-frequency type.

"A comprehensive analysis of all deformation data (EDM, electronic tilt, dry-tilt, and levelling) now clearly shows asymmetric behavior of the volcanic edifice. Most deformation data have been gathered in the NW sector and the least in the SE sector (where only one electronic tiltmeter is sensing deformation now, apparently with considerable electronic or geologic drift). However, comparison of the data sets suggests strongly that movements are most pronounced in the NW sector, somewhere between the main crater Arenas and the dormant crater La Olleta, on the W flank.

"dry-tilt station Molinos (3.2 km N of Olleta and 5.0 km NW of Arenas) recorded a fluctuating change of several tens of microradians, corresponding to upward movement in a direction E of Olleta (toward the summit) between mid-April and mid-June. There was a general agreement with this sense of deformation at Inderena electronic tiltmeter (1.1 km NE of Molinos dry-tiltmeter). The start of this anomaly corresponds roughly to the reactivation of seismicity (20 April). The deformation at Molinos reversed to downward movement around 17 June, while seismic activity continued.

"A comparison of all EDM lines also reveals that deformation is concentrated in the NW sector (line Arbolito). New EDM lines and dry-tilt stations are now being installed, to allow for more resolution of the deformation pattern.

"Ash emission seemed to be more or less continuous at one of the fumaroles in the crater. Samples of ash, taken from recent falls, were examined under the microscope. No signs of juvenile material were evident. The vapor column, although quite dense, was not seen at heights above 500 m. No correlation has been found yet between the fumarolic activity and the amplitude/period of the harmonic tremor."

Activity during 11 July-13 August. "For the first time in the present period of increased activity (since 20 April), strong ash emissions occurred on 20 and 29 July. Harmonic tremor changed its pattern, especially after the prolonged ash emission of 29 July, becoming stronger and changing spectra. Deformation slowed after the 20 July emission. Under the microscope, ash samples from both emissions showed no signs of juvenile origin.

"The first emission, on 20 July at 0600, was short (~2 minutes) and more vigorous, rising to >4 km above Arenas Crater. It also contained a visible amount of vapor. The 29 July emission, detected at 0630, lasted longer. Pulses of ash were observed between 0635 and 0758, but were less vigorous than on 20 July and vapor was not visible. The ash cloud spilled over the crater and down the W flank (over the Farallones). After 0758 the tremor decreased. Ash emission continued, at a lower rate and more steadily, until about noon. Dominant dry-season winds carried ash from both emissions at least 30-40 km to the W, but no change was observed in W flank streams, although the snow was covered with ash.

"Vigorous phases of both emissions were accompanied by strong tremor that partially saturated instruments. The most interesting feature of the seismic activity, however, preceded both ash emissions. Swarms of small B-type events, most visible at Refugio station 3.6 km W of Arenas Crater, started 7-8 hours before the emissions. Attempts to link this station by telemetry have been impeded for several weeks by high levels of gas and lack of accessible shelter in the area. Both emissions were also preceded by shallow seismic events occurring 20-120 minutes in advance.

"Both emissions produced 4-6 Hz tremor. After the 29 July emission, several tens of hours of tremor with a dominant component of 0.6-1.0 Hz were recorded at Recio (4 km SE of Arenas) and less strongly at Olleta (4 km W). Following several days of intermittent tremor, the tremor frequency increased again after 7 August to 4-5 Hz and showed strong variations in amplitude, which could be correlated between the three nearest telemetering stations Rubi (4.6 km N of the crater), Olleta, and Recio.

"During a period of low-amplitude tremor on 8 August, several shallow events and one A-type swarm (>20 events) occurred, followed by ash emission. Ash emissions that were considerably smaller than those of 20 and 29 July were seen 9 and 10 August. Activity declined 11-12 August, but increased again on the morning of the 13th, when alternating emissions of pulsating ash plumes and vapor were observed.

"Deformation started to slow simultaneously with the 20 July emission, but still showed changes reflecting inflation at most stations."

Activity during 13 August-12 September. "No more strong ash emissions occurred after 29 July, although prolonged and weak emissions have been reported. We assume that the record of weak emissions is incomplete due to weather conditions and frequent snowfall.

"After a strong phase until about 20 August, the amplitude of harmonic tremor decreased, but tremor remained continuous and maintained 5 Hz as a dominant frequency. Occurrence of seismic events reached a mimimum 17-23 August, with no events above the tremor level for >40 hours. Seismicity then increased again, reaching a maximum of ~120 events on 9 September. During this maximum, several long-period events were recorded, quite similar to but smaller than those observed before November 1985. About half of the remaining events were of high frequency (A-type) and most of the rest were B-types.

"The pattern of deformation changed in comparison to all previous months of 1986. By the end of August, most dry-tilt stations began to show fluctuating changes, with amplitudes 2-4 times as large as those observed since December 1985. Cumulative resulting vectors pointed approximately to the summit area."

"After peaking in the first half of September, most indicators of activity have been at lower levels. The seismic event count (120 on 9 September) did not exceed 25 events/day. Long-period events, however, continued to occur at about the same rate as before. The most recent large ones were recorded on 5 October (duration ~35 seconds) and 11 October (~25 seconds). Harmonic tremor continued at intermediate levels, but was less stable in amplitude. Long-period and shallow seismic events tended to occur more frequently during periods of low tremor amplitude. Deformation, as measured by dry and electronic tilt, has diminished. September deformation rates were the highest since 1985, but, similar to dry-tilt data from October-November 1985, recent changes were fluctuating rather than uniform in character. The highest SO2 emission rate during the report period was 7,200 t/d, measured on 18 September. As of 11 October, the rate was 1,000-2,000 t/d. Ashfall has been observed on many days. None of the analyzed samples were of juvenile origin.

Activity during 11 October-11 November. "A swarm of tectonic earthquakes on 28 October was the major recorded seismic activity for the report period. The swarm lasted 7.5 hours, and included >180 events with durations >= 5 seconds. Maximum magnitude was 2.9. The events were centered ~3 km W of Arenas Crater and 1 km NE of Olleta Crater, at ~5 km below summit level. In comparison, the much smaller swarm that initiated the present period of higher activity on 20 April occurred SE of Arenas Crater, at a shallower depth (~3 km).

"Long-period seismic events were less frequent than during September. Low-frequency seismic events continued at about the same rate and most still appeared to be centered near the SE station (Regio). Since about 6 November the seismic station nearest Arenas Crater (Refugio, 2.8 km to the W) recorded an increase in shallow seismic activity.

"Harmonic tremor was the lowest of any period since 4 May. Several times, the signal declined to background level for hours or days. As of early November, tremor typically lasted for a few hours, often beginning after low-frequency or shallow seismic events then slowly declining. At times, the increase or decrease in signal intensity was not simultaneous at all stations, suggesting that there was more than one tremor source.

"The highest SO2 emission rate, 5,400 t/d, was measured on 31 October during a tremorless period. The average emission rate was ~3,000 t/d. Evidence of ash emission was less than during the previous month.

"After relatively strong fluctuating changes in September, deformation was weaker and less regular. The resultant tilt vectors pointed less consistently to the summit area. Some changes in deformation measurements may have been due to the effects of heavy rainfall."

"Activity remained at about the same level as . . .11 October-11 November, but showed different character and location. Shallow (non-tectonic) seismic events, which increased at the end of October and early November, were more frequent than during any previous month in 1986. From measurements of station-to station amplitude ratios this high-frequency activity appeared to be W of the volcano. Low-frequency events during earlier months had been centered more to the E, suggesting a possible relationship with tectonic-type swarms in April and late October. During the same period, the frequency of shallow (explosion-type) seismic events increased. Harmonic tremor remained at low levels with occasional increases correlating with increases in shallow seismic activity.

"The crater fumarole generally emitted very small steam plumes, but some plumes reached more than 1 km when wind speeds were low. Hot springs 10 km NW of Arenas Crater showed a decrease in pH and an increase in temperature for the first time since the pre-eruption period of 1985. Ash emission was the lowest since 4 May. COSPEC measurements were infrequent due to poor weather conditions but average SO2 emissions were between 2,000 and 3,000 t/d. The latest measurement, on 10 December, was 1,700 t/d."

"There was no significant change in the average level and character of activity.

"Shallow B-type events still dominated the seismic activity, remaining as frequent as during the two previous months. A small swarm of tectonic events (A-types) on 22 December, centered SE of the main (Arenas) crater, marked an increase in D-type occurrence, which averages more than 50 events/day. Only a few brief (hours) and low-amplitude episodes of harmonic tremor were recorded. Deformation measurements yielded only minor, fluctuating changes.

"The hot springs NW of the main crater had started to show changes in pH and temperature the previous month, and these values continued to fluctuate. The vapor column remained vigorous, fluctuating in height and intensity, and pulsating. Only a few reports of minor ashfall were received. The rate of SO2 emission (measured by COSPEC) decreased; the average was ~1,400 t/d, the highest values 2,700 t/d. After 2-3 months with very little snowfall, some glaciers were showing signs of strong melting and fracturing.

The activity level fluctuated during the 11 January-11 February report period, but remained similar to the previous 30 days. Shallow, low-frequency seismic events dominated activity but a few long-period events occurred (for the first time since November) during the last 2 weeks of the report period. Larger tectonic and low-frequency events (M > 1.0) occasionally occurred at the SE limit of the active zone, 2-4 km from Arenas Crater. No clearly discernable tremor was recorded.

Electronic tilt was the quietest since the first observations in October 1985 but increased on 7 February. Very small fluctuating changes were seen from dry-tilt measurements until the end of January when small (less than a microradian/week) but consistent deformation began.

COSPEC data indicated that the SO2 emission rate remained fairly stable, averaging 1,400 t/d. However, the H2O content in the column again seemed to have increased significantly. There was no evidence of ash emissions.

No major change in activity level has been observed during the 11 February-11 March report period but a correlation between measured data and crater activity was established. Most dry-tiltmeters began to show stronger variations (maximum amplitude 10 µrad) by the end of January. Electronic tiltmeter 'Inderena' (5.9 km NW of Arenas Crater) began to record inflation after 7 February that increased between 15 and 19 February to 0.5 µrad/day, the strongest anomaly since late September 1986.

After deformation flattened out on 20 February, several small ash emissions occurred 20-25 February. COSPEC values increased to a maximum of 4,900 t/d during this period but there was no recognizable correlation with the seismicity. The number of shallow, small-magnitude B-type events remained about the same since December 1986. Only harmonic tremor increased between 20 and 25 February. Its shallow source was evident from amplitude relations.

Seismicity, particularly shallow B-type activity, began to decline slightly on 20 March. In the following weeks, long-period events were more frequent, and high-frequency (A-type) events shifted N towards Arenas Crater and became more shallow. About 1-2 events were recorded daily. Tremor was weak but more continuous than during the previous report period.

Deformation, consisting mainly of NW-SE fluctuations, remained at slightly elevated levels, comparable to September/October 1986. Changes were recorded more strongly on lower-altitude stations. From 4-10 April an electronic tiltmeter (Inderena) 5.9 km NW of Arenas Crater recorded 2-3 µrad/day of eastward inflationary movement. Other tiltmeters recorded only minor amounts of inflation.

SO2 emission, measured by COSPEC, declined to an average of 620 t/d (maximum 1,060 t/d on 27 March). There have been no reports of ash emission since late February. About 3 April, [hundreds to thousands of] dead [migratory] birds and small mammals were found 10-20 km from the crater, mostly to the NE (downwind). Preliminary analyses showed sulfur emission rates and rain acidity to be well below peak values, but biological analyses of animals revealed signs of carbon monoxide effects [see also 12:05].

Seismicity continued at moderately high levels in April; 920 low-frequency and 172 high-frequency events were registered. The tremor signal has remained weak compared to 1986. Seismic activity peaked on 16 and 17 April when 80-90 events were recorded each day [see also 12:05]. One week later, deformation measurements showed changes of as much as 5 µrad/day. SO2 levels, measured by COSPEC, averaged 1,000-1,200 t/d.

Activity increased again after several months at low to moderate levels. Swarms of high-frequency earthquakes occurred 21 and 22 May, 3-6 km SW of the active crater (Arenas), at an average depth of ~6 km. 125 events were registered above the count threshold, with a maximum magnitude of 2.0. Low-frequency events (B-type) remained stable. Slightly shallower seismic events were recorded following the swarm, similar to observations after the previous swarm (16 April).

Harmonic tremor was generally absent or just above background, but increased suddenly on 9 June at 2007 to levels close to saturation, similar to the tremor activity recorded for months after the 4 May 1986 ash emission. Ashfall was first reported more than an hour later at 2115 (at Cerro Gualí) but visibility was poor, so the onsets of ash emission from Arenas Crater and tremor may have been simultaneous. Fine ash fell 40-50 km to the W, and by the afternoon of 10 June, 3.5 cm had accumulated 6 km W of the volcano. Initial microscopic inspection indicated no fresh glass in the ash. A helicopter overflight 11 hours after activity began revealed no impact craters or other evidence of ejection of large tephra. Minor melting of summit-area ice caused small increases in the flows of W flank rivers, but no evacuations were necessary. No pyroclastic flows were observed. After the first day of activity, ash emission declined irregularly, and had ended four days later. Tremor declined steadily, with no apparent correlation between its amplitude and observed fluctuations in ash emission.

COSPEC data showed that SO2 emission rates increased slightly for a few days after the May earthquake swarm, to a maximum of 2,300 t/d. Weather clouds and ash hampered COSPEC measurements during the ash emission period. Values ranged from 850 t/d on the first day of the activity to 3,600 t/d the second and fifth days, but because of masking effects of the ash, these are minimum values.

Deformation measurements did not yield values that correlated clearly between stations or with seismic and eruptive activity. One dry-tilt station N of the summit showed a change of 12 µrad just before the May seismic swarm, and a SE-flank electronic tilt station recorded ~11 µrad of inflation in the week before the swarm.

The deaths of birds mentioned in 12:03 [and originally in 12:04] were observed 2-4 April, 25-26 km NE of Arenas Crater (in Casabianca, Villahermosa, and Herveo) and at 4,200 m altitude 8 km S of the crater (at Laguna Verde). An estimated 2,000 birds died in that 3-day period, during which clouds and rain were almost continuous, and residents reported a constant rotten egg odor. Most of the affected birds, generally migrants en route to North America, fell to the ground between the hours of 0300 and 0400. Some were still alive but had evident breathing problems, and some survived several days before dying. Seventeen birds were taken to the Instituto Colombiano de Agricultura for study. All had lung damage, and autopsies suggested that they died from carbon monoxide poisoning. There were no reports of ill effects to humans. Similar bird mortalities were reported at the end of the 19th century during a period of increased fumarolic activity at Ruiz.

Low- and high-frequency earthquakes continued at moderate levels after 16 June. Tremor fluctuated but remained weak, reaching a maximum amplitude of 2 mm at the Olleta reference station (4 km W of Arenas crater). The rate of SO2 emission measured by COSPEC reached 5,640 t/d on 17 June, the highest value of 1987. Emission rates then declined to an average of 1,000 t/d for the rest of the month. Little change in deformation was measured.

No new eruptive activity has been reported since the 4-day ash eruption that started 9 June. Seismic activity declined after the eruption and has continued at moderate levels since mid-June. During July, 162 high-frequency, 445 low-frequency, and 68 shallow explosion events were recorded. The actual number of events was obscured by tremor that filled the records on some days. Major high-frequency swarms occurred on 17 July (57 events) and the night of 31 July-1 August (120 events). Strong tremor occurred 14-16 and 23-26 July, with peak-to-peak amplitudes of 9-10 mm. Additional earthquake swarms were recorded on 6 August (high-frequency) and 9 August (low-frequency).

Deformation measurements were more stable in July than in June. On 7 July, all stations showed inflationary trends, with a maximum value of 10 µrad. The SO2 content of the plume as measured by COSPEC decreased to the lowest values yet measured, 170 t/d.

August data are from the Boletín Informativo, published by the Observatorio Vulcanológico de Colombia. Energy release from the high-frequency earthquake swarm of 31 July-1 August was the fourth largest since the seismic crisis of November-December 1985. Maximum magnitude of swarm events was 2.2. The number of both high- and low-frequency events increased slightly in August, to 299 and 585 from 162 and 445 in July, but shallow explosion events declined from 68 in July to 50 in August. All of the August high-frequency events were centered in a zone 3-6 km S-SE of the crater, about 2/3 with focal depths 2.5-4 km below the summit. The remainder were shallower. Swarm events had epicenters 3.5 km S of the crater with similar depth distributions. After the swarm, seismic energy release accumulated in August at a rate similar to that of July. Tremor amplitude and period varied considerably, but energy was generally lower during the month. Highest amplitudes were recorded on 17 August, associated with an apparent minor ash emission. A small amount of ash was also visible in the plume at about 1000 on 22 August; poor weather prevented observations during other periods of increased tremor. Measurements of rates of SO2 emission were hampered by large variations in wind direction and velocity; values varied from ~450 to 2,600 t/d. Deformation measurements showed no major changes.

A small ash emission on 10 September dispersed ash 4-5 km SW. Seismic activity associated with the ash emission was the strongest of the month and saturated the records of stations close to the crater for more than 10 minutes. A strong deformation change was recorded by the Refugio station (3.6 km W of Arenas Crater) a few minutes before the ash emission, but was not detected at other deformation stations.

Seismic activity in September was characterized by an oscillatory tremor signal that fluctuated between high and moderate levels. Three more seismic episodes similar to that of 10 September occurred during the month but did not saturate instruments. A swarm of high-frequency events occurred 25 September. September totals were 154 high-frequency and 533 low-frequency events. Deformation changes were minor. The rate of SO2 emission grew to an average of 2,000 t/d, compared to the year's average of 1,000 t/d.

A small ash emission occurred on 25 October at 0946. Seismicity had begun to migrate toward the crater in mid-October, and the activity was accompanied by a sudden increase in harmonic tremor that lasted for ~15 minutes. Tremor was otherwise very weak during October. During the month, 301 high-frequency, 593 low-frequency, and 125 shallow (explosion) events were recorded, up from 154, 533, and 15 in September. Changes in deformation remained small to moderate. Rates of SO2 emission were higher than in any previous month of 1987, with median values of close to 2,000 t/d.

Seismic activity continued at high levels in December and January. During December, 423 high-frequency and 714 low-frequency earthquakes were recorded; a swarm of 101 high-frequency earthquakes occurred on the 11th. In January, low-frequency earthquakes increased to 1540 while high-frequency events decreased to 280. Shallow earthquakes were steady, with 62 recorded in December and 65 in January. The amount of seismic energy released increased slightly. Tremor signal was low and no ash emission was detected.

COSPEC measurements of SO2 ranged from 450 to 3,000 t/d at the end of January, when a slight increase in SO2 content was recorded in the plume. Deformation measurements that month showed minor to moderate changes. Most (70%) of dry-tilt changes were <8 µrad; Molinos Station (on the NW flank) continued to show instability with tilt variations reaching 15 µrad. The telemetric tilt data did not show any significant changes.

February data are from the Boletín Informativo, published monthly by the Observatorio Vulcanológico de Colombia. The number of high-frequency events increased significantly in the second half of February, totaling 707 for the month, as compared to 280 in January (figure 7). Hypocenters, at 1-5 km depth, were clustered in three zones: near Nevado de Santa Isabel (~10 km SW of the active Arenas Crater) on 10 February; to the NE at Nevado del Cisne (~6 km SW of Arenas) on 21 February; and an especially active sequence at Arenas 12-22 February. Low-frequency events declined slightly to 1,362, from 1,540 the previous month, and did not parallel the late February increase in high-frequency shocks. A few more shallow earthquakes were recorded in February (74) than in January (65). Tremor remained at minimal amplitudes until the end of the month, when brief episodes of long-period (0.7-1 second) tremor reached maximum peak-to-peak amplitudes of 7 mm. The rate of SO2 emission measured by COSPEC ranged from ~1,000 to 3,000 t/d and averaged 1,843 t/d. Deformation changes, monitored by dry and electronic tilt and electronic distance measurements, appeared to be small.

Figure 7. Daily number of recorded earthquakes at Ruiz, 20 July 1985-31 March 1988. Larger eruptive episodes are labeled. Courtesy of INGEOMINAS.

Ash emission began on 22 March at 0935, the day after a long-period earthquake swarm. Activity briefly became more vigorous on 25 March before declining that night. Eruption column heights did not exceed 2-3 km and were usually <1 km. Ash fell to the NW, where 2 mm accumulated in Manizales (~30 km from the crater) and 15 mm were deposited closer to the volcano. No fresh magma was observed in the ash. Tremor increased rapidly 20 minutes after the onset of ash emission, remained at a high level for the next six days, then decreased slowly before reaching a minimum on 29 March. Civil Defense officials declared an orange alert and ordered the evacuation of the area within 10 km of the crater. Several hundred people left the hazard zone, but the press reported that at least 1,000 chose to remain. The warning was reduced to a yellow alert on 27 March, signaling that the activity was not imminently dangerous.

The number of recorded seismic events was 220% higher in March than in February. Most events were centered in two zones, one beneath the active Arenas crater (sometimes called Cumanday) and the other 3.5 km from the crater on the SW flank. Depths ranged from 0.15 to 5 km. Maximum daily seismic energy release was 124 x 106 ergs (figure 8). Rates of SO2 emission reached 7,700 t/d (figure 9), the highest measured since September 1986. The average daily rate in March was 2,800 t/d (figure 10) but values declined after ash emission began, a pattern similar to that associated with the June and October 1987 ash emissions. No significant changes were observed in deformation data.

Small ash emission episodes that lasted ~2-5 minutes were frequent in the second half of April, as seismicity increased significantly. Ash fell ~10 km NW and SW of the summit crater, troubling cattle on nearby farms.

The number of high-frequency events increased significantly during the last six days of April, reaching a total of 2,683 for the month. The number of low-frequency shocks also increased in April, totaling 2,303. Epicenters were dominantly aligned NE-SW in two principal zones; one beneath the summit (Arenas Crater) at 1-5 km depth, the other 3 km to the SW (between Arenas and Nevado del Cisne) at 3-5 km depth. Maximum daily seismic energy release was 2.04 x 108 ergs on 29 April (figure 11), when 338 high-frequency and 247 low-frequency events were recorded. Pulses of tremor that began suddenly and reached high amplitude occurred almost daily, but declined very quickly (after 2-5 minutes). Several of the tremor pulses were associated with small emissions of ash that fell near the crater. SO2, measured by COSPEC, declined at the beginning of April then increased to an average of 3,000 t/d (figure 12), very close to March values. Deformation data continued to show no significant changes.

In May, minor ash emissions were associated with pulses of high-amplitude tremor that lasted a few minutes at the beginning of the month, and had longer durations after 21 May. Seismicity decreased in May but remained at high levels. The number of high-frequency events declined from 2,683 in April to 1624 in May, and low-frequency shocks dropped from 2,303 to 1,902. Hypocenters were concentrated S and SW of Arenas Crater at depths of 1-5 km. The maximum daily energy release (1.2 x 108 ergs) occurred 31 May when 42 high-frequency and 306 low-frequency earthquakes were registered. Relatively good weather and consistent winds (1-5 m/s velocity) allowed SO2 measurements by COSPEC on 13 days. The maximum measured rate of SO2 emission was 5,190 t/d on 13 May, with a monthly average of 2,435 t/d. Most of the highest SO2 values corresponded with ash emissions and increases in the tremor signal. No significant changes were registered by electronic or dry-tilt.

Several small brief ash emissions occurred daily during June, all associated with increased harmonic tremor. Ash fell to the W, usually <10 km from the vent. High-frequency earthquakes increased to 2,362 in June, while low-frequency events decreased to 1,758. A significant high-frequency swarm (273 events) occurred on 15 June (figure 13). There were 212 shallow events in June and daily energy release tended to increase, continuing a trend that began in late 1987 (figure 14). Average SO2 content was 1,700 t/d (measured by COSPEC), a decrease from May's average of 2,435 t/d. Deformation measurements showed no significant changes.

Figure 13. Daily number of recorded seismic events of high (solid line) and low frequency (dashed line), June 1988. Courtesy of the Observatorio Vulcanológico de Colombia.

Several ash emissions in July deposited 2 mm of lithic ash as far as 5 km from the crater. COSPEC measurements indicated an increase in SO2 emission during the second half of the month (figure 15) with highest contents recorded on 18 July (5,560 t/d) and on 26 July (4,850 t/d).

Figure 15. July 1988 SO2 emissions from Ruiz, as measured by COSPEC.

Seismic activity also increased in July. The highest energy earthquake swarm in July occurred at the beginning of the month near Nevado Santa Isabel, ~5 km SW of the volcano. At month's end, most activity occurred 2 km SW of Arenas crater at depths of 0.5-5 km. Short-duration tremor was also measured. Only minor deformation changes were detected.

Small, brief ash emissions similar to those of July continued during August. Seismic energy release and the number of low-frequency earthquakes increased slightly (figure 16). Some clusters of high-frequency events occurred, especially ~2 km NE and SW and 4 km SE of the crater. No significant deformation was recorded. SO2 emissions (figure 17) averaged 2,139 t/d.

Figure 16. Number of seismic events (high- and low-frequency)/day, June 1985-mid-September 1988, courtesy of the Observatorio Vulcanológico de Colombia. The 11 September and 13 November 1985 eruptions are indicated by arrows.

Figure 17. Daily SO2 flux as measured by COSPEC, mid 1986-early September 1988. Courtesy of the Observatorio Vulcanológico de Colombia.

The number of high- and low-frequency seismic events increased in September. Although seismic energy release declined slightly from August, energy release has risen substantially during 1988 (figure 18). Most September seismicity was centered ~4 km SE of the crater at depths of 0.5-7 km. A high-frequency earthquake swarm 2 km SE of the crater occurred 18 September. Brief increases in tremor were associated with small ash emissions. Tremor amplitude was at one of the year's highest levels. SO2 emissions averaged 2,870, up from 2,139 t/d in August (figure 19). No significant deformation was recorded.

Figure 18. Seismic energy release at Ruiz, June 1985-September 1988. Periods of tephra emission are labeled. Courtesy of the Observatorio Vulcanológico de Colombia.

Figure 19. Daily SO2 flux as measured by COSPEC at Ruiz, mid-1986 to September 1988. Courtesy of the Observatorio Vulcanológico de Colombia.

The number of earthquakes and seismic energy release remained at high levels during October, continuing the pattern of vigorous seismicity that began to build in late 1987. Tremor amplitude declined slightly. Fluctuations in tremor amplitude were associated with small explosions that briefly emitted ash. A swarm of 212 high-frequency events was recorded 1 October from 1,410 to 1,450. The month's two strongest shocks (M 2.3) occurred on the 9th and 12th. Several of the locatable events were scattered SE of the volcano, while others were centered near the crater at depths of 1-4 km (figure 20). No significant deformation was recorded.

Figure 20. Epicenters of 49 seismic events at Ruiz, October 1988. Courtesy of the Observatorio Vulcanológico de Colombia.

Energy released in November by high- and low-frequency earthquakes and volcanic tremor remained similar to October values. A tremor episode on 23 November was associated with fumarolic activity and a brief landslide in the canyon of the Río Azufrado, 2 km NE of the crater (figure 21).

Figure 21. Epicenters of high-frequency earthquakes, November 1988, and the position of the Palestina Fault. Courtesy of the Observatorio Vulcanológico de Colombia.

Although the tremor episode lasted only a few minutes, it produced one of the largest displacements of the energy release curve since 1986. A slight increase in seismicity followed for the next few hours. The month's main high-frequency activity was centered parallel to the regional Palestina Fault, occurring in two clusters. Depths were between 0.5 and 6 km. No significant deformation was measured during the month.

The number and energy release of high-frequency seismic events at Ruiz continued to decline in December, while low-frequency seismicity was similar to previous months (figure 22). Total seismic energy release remained substantially elevated (figure 23). There were two main clusters of high-frequency events, one roughly 4 km E of the crater at ~2.5 km depth, the second 3 km SW of the volcano at ~2 km depth. Brief increases in tremor intensity occurred more often in December, but their energy was low. No significant deformation was measured during the month. The rate of SO2 emission averaged ~1,220 t/d.

During January, total seismic energy release increased (figure 24a), as did the number of low- and high-frequency events (figure 24b). Most of the high-frequency seismicity was centered in an elongate NW-SE zone between Arenas Crater and Nevado del Cisne (figure 25). Focal depths generally ranged from 0.5 to 4.5 km. Swarms of long-period events occurred on the 17th and 29th, but their energy release was relatively low. Brief increases in the intensity of shallow tremor were usually associated with small ash emissions. A different type of tremor occurred 2-3 January; with a monochromatic signal, a dominant period of 0.35-0.45 seconds, and low energy. During the month, deformation did not exceed 10 µrad except at Molinas station (4 km NW of Arenas crater), which was affected by rainfall.

Daily seismic energy release showed a large increase in February, with the highest telemetric reading since monitoring began in 1985. The number of seismic events was similar to recent months. Long-period and high-frequency events dominated the seismicity. Tremor generally remained at lower levels, with brief pulses of high-amplitude, low-energy tremor.

Deformation changes (measured by dry-tilt and a short level-line) were generally only small to moderate. Electronic distance measurements were stable at the Refugio and Recio stations (~1.1 and 2.2 km from the summit, respectively). Nine SO2 measurments were made by COSPEC during the month. One reading (on the 9th) showed an emission rate of 8,949 t/d. Mean rates in December and January were 1,220 and 1,800 t/d.

Seismic energy and the number of high-frequency events decreased during March continuing a general long-term decline. Short pulses (averaging 5 minutes) of low-energy tremor with periods of ~0.25-0.30 seconds were associated with small ash emissions. Dry and electronic tilt and short leveling vectors showed little change. SO2 emissions measured by COSPEC varied from 900 to 4,500 t/d with a monthly average of 1,921 t/d (see figure 26). Emissions were blown SE.

A swarm of more than 500 high-frequency events on 13-14 May had the second highest energy release since telemetric monitoring began in 1985. Seismic energy release increased 3.5x during the swarm, centered <2 km below Arenas crater (figure 27). Brief pulses of low-energy tremor and a small number of long-period events were also recorded during the month. Deformation (dry and electronic tilt) was low. SO2 emissions continued to decrease, to an average of 1,046 t/d in May.

Seismic energy release has been at increased levels since about February 1988. A sharp increase in seismicity began on 24 June 1989 with a felt earthquake (M 3.1) in Arenas crater. The next day, a shallow swarm of high-frequency events (also in Arenas crater) began at 1130 and continued for 1 hour. From 0100 to 1100 on the 26th, another high-frequency swarm was centered at 4 km depth, 3 km W and SW of Olleta crater (Olleta is roughly 5 km W of Arenas crater). Late that evening, a shallow high-frequency swarm began in Arenas crater, followed by strong tremor associated with a small ash emission that deposited 1 mm of ash, 4 km from the crater. The press reported that the civil aeronautics board issued a warning to airline pilots to avoid a 60-km area around the volcano. Tremor gradually diminished, disappearing on 28 June. SO2 emission was moderate during June. Dry and electronic tilt did not show significant changes. As of 10 July, a yellow alert remained in effect for population within a 10-km radius of the volcano.

After a sharp increase in seismicity on 24 June and a small ash emission 2 days later, seismicity gradually decreased in late June. High- and low-frequency events stabilized at 200-300/day by the end of the month (figure 28), and tremor was almost absent. A depression, 150-200 m in diameter and 80 m deep, apparently formed on 26 June ~50-100 m SW of the principal (Arenas) crater. It probably developed because of high-pressure gas emission and destabilization of the walls of Arenas crater.

Figure 28. Daily number of high- and low-frequency events at Ruiz, July 1989. Courtesy of the the Observatorio Vulcanológico de Colombia.

Increased seismicity since mid-August was followed on 1 September by a summit phreatomagmatic eruption that deposited tephra NE of the crater and generated a damaging mudflow.

Vigorous earthquake activity was recorded during August, with numerous A-and B-type events, continuing a trend of increased seismicity that began about February 1988. The month's strongest high-frequency seismic energy release was on 19 August, from a swarm of events centered S of Arenas Crater (figure 29), the main vent of the fatal 1985 eruption. The rate of SO2 emission generally increased in August, oscillating between 416 and 3,800 t/d, for an average of 1,762 t/d (figure 30). Little change was noted in August dry-tilt data and no major changes were detected by electronic tilt instruments. However, the N-S component of the Refugio tilt station (2.3 km WNW of Arenas Crater) has shown a gradual inflationary trend since May 1987 that totaled ~64 µrad by the end of August 1989; the E-W component has registered only minor fluctuations (figure 31).

Figure 29. Epicenters of 35 high-frequency earthquakes near Ruiz, August 1989. Courtesy of the Observatorio Vulcanológico de Colombia.

Figure 30. Monthly means of rates of SO2 emission measured by COSPEC at Ruiz, July 1986-August 1989. Courtesy of the Observatorio Vulcanológico de Colombia.

Figure 31. N-S (top) and E-W (bottom) components of electronic tilt recorded at the Refugio station, March 1987-August 1989. Courtesy of the Observatorio Vulcanológico de Colombia.

Long-period earthquakes increased sharply in the week before the eruption. Tremor (3 mm peak to peak) began late 30 August, associated with ash emission, and ash was emitted again the next day. COSPEC SO2 data provided no obvious short-term warning, with measurements ~18 hours before the eruption yielding values of roughly 2,500 t/d. The day before the eruption, increasing high-frequency seismicity was centered in two zones, NW and just S of Arenas Crater. Staff from the Observatorio Vulcanológico de Colombia met with government officials on 31 August to express concern about the possibility of an eruption.

The phreatomagmatic eruption began at about 0215 on 1 September as seismicity increased. Witnesses saw incandescence in the crater and forest guards reported the fall of bean-sized pumice. Tremor began to increase about 0240, and by 0330 stations more than 10 km from the crater were saturated; 40 cm2 of reduced displacement were recorded during the most vigorous activity. The exact timing of cessation of activity was uncertain, but the eruption had ended by 0900. Pumice and lithic fragments were deposited along a sector from N30°W to N80°W, toward the Pacific Ocean (figure 32). The maximum measured tephra thickness was 5 cm (at Inderena) 5 km N60°W from the crater. The tephra deposit was 2 cm thick in Manizales (~30 km from the crater), composed of lithic fragments, pumice, and crystals of plagioclase, quartz, ferromagnesian components, and biotite, covered by a layer of fine dust. The activity generated a lahar, rich in large (>2 m) andesitic clasts, with coarse sand and sporadic pumice fragments, that advanced 7 km NE down the Azufrado river valley and destroyed a bridge along the Manizales-Murillo road. At the bridge site, the lahar was 5 m thick and 20 m wide.

Figure 32. Preliminary isopach map of the 1 September 1989 eruption. Courtesy of the Observatorio Vulcanológico de Colombia.

The Nimbus-7 satellite's TOMS showed little or no enhancement in SO2 values as it gathered data over the Ruiz area at about local noon on 1 and 2 September. Heavy weather clouds obscured the area, and no plume was detected on weather satellite images.

The Observatorio Vulcanológico de Colombia and the Comité Regional de Emergencia declared a red alert at 0450 and triggered sirens along the principal rivers. Evacuation began from 16 communities (total population <5,000) within 10 km of the crater. As eruptive and seismic activity had declined, the alert status was downgraded to orange at about 1500 and residents were allowed to return to their homes.

The next morning, the vigorous 200-300-m gas plume that was emerging from the summit did not appear to contain any ash. Rates of SO2 emission measured by COSPEC were roughly 2,500-3,500 t/d the day after the eruption, and had risen to 6,000 t/d on 3 September; similar increases have been observed after previous eruptive episodes. The pH of hot springs 7 km NNE and 11 km NW of the crater (at Aguas Calientes and Hotel Termales) had dropped from previous values of 1.2-1.5 to 0.84 and 0.94, but temperatures remained unchanged. Changes in temperature, pH, and sulfur/chlorine ratios had been detected at the same hot springs in late 1986. A M 3.3 earthquake centered under the crater occurred on 5 September at 1845. This event was felt and subterranean noises were heard in the 10-km evacuation area. As of 15 September, no additional strong eruptive activity had occurred. Strong seismicity was continuous, dominated by high-frequency earthquakes centered 6 km W of Arenas Crater at 4-6 km depth. Some of the events have been felt near the volcano.

Although the number of A-type events increased in September, the number of B-type shocks decreased from August values. Most September epicenters fell along a roughly NE-SW trend (figure 34). Several events were felt on the volcano. Seismic energy release was highest on 14 September when a swarm of A-type events occurred, centered on the W part of the summit. dry-tilt measurements showed deflation, while electronic tilt measurements showed no significant changes.

Figure 34. Epicenters of 97 high-frequency earthquakes near Ruiz in September 1989. Courtesy of the Observatorio Vulcanológico de Colombia.

In October, seismic energy release and the number of high- and low-frequency events decreased to the lowest level since early 1988 (figure 35). High-frequency events were centered under and N of the crater. Tremor was unstable during October, with frequencies oscillating between 5 and 1.8 Hz during the first days of the month. Near the end of the month, tremor frequency reached 1.6 Hz with a reduced displacement of 0.38 cm2. SO2 emission ranged from 174 to 2,648 t/d, averaging 1,345 t/d during the month. Deformation measurements (electronic and dry-tilt) showed no significant changes.

Seismicity . . . remained at moderate-low levels in November. Seismic energy was highest on the 22nd, when 694 low-frequency events released 1.8 x 108 ergs. High-frequency seismicity increased only slightly in the second half of the month, with events located N and SW of the active crater at 0.5-6 km depths. Low-frequency tremor was often recorded through the 15th. Brief tremor pulses occurred throughout the month, particularly in the second half, when the majority were associated with small explosions that deposited ash around Arenas crater. SO2 emissions averaged 1,770 t/d (corrected for wind conditions), a slight increase from last month's average. Dry and electronic tilt showed no significant changes.

Seismicity dropped in late September, reaching the lowest level since early 1988 in October, and remained at low to moderate levels in November. Seismic activity remained relatively stable in December, with no significant changes in the daily number of high- or low-frequency earthquakes. The energy release of low-frequency events increased slightly toward the end of the month, while the energy released by high-frequency events decreased slightly. High-frequency events were centered in three zones; N of, beneath, and scattered around Arenas Crater at 1-6 km depth. Swarms of these events occurred on the 16th and 18th. Short pulses of low-energy tremor were associated with small ash emissions, one of which deposited a thin layer of ash over Manizales (30 km from Arenas crater) on the 27th. Dry and electronic tilt and short leveling vectors showed no significant variations. SO2 emissions measured by COSPEC were moderate, averaging 1,800 t/d.

Seismicity was at low levels during the first ten days of January, then the number and energy release of high-frequency events increased significantly. Swarms of high-frequency events, mostly centered in a zone ~6 km N of the crater at ~6 km depth, caused four energy release peaks (figure 36). A small number of low-frequency events were recorded during the month. Short pulses of low-energy tremor were not associated with ash emission. Deformation measurements showed no significant changes in January. Five SO2 measurements during January yielded an average of 980 t/d, only about half that of the previous month.

Seismic energy release and the number of earthquakes were at low levels in February. A swarm of low-energy, high-frequency events occurred NW of Arenas crater at 6.5 km depth on 7 February. Pulses of low-energy tremor were also detected. The average measured dry-tilt change in February was only 4 µrad. Variations in electronic tilt (at the Refugio station) were associated with tremor (1.5-2.0 km depth) 14-16 February.

The number of earthquakes and seismic energy release remained low in March. Located events were centered W and SW of the crater. The strongest recorded earthquake (M 2.1) occurred 21 March. Only a few short pulses of low-energy tremor were recorded, except for a high-energy episode on 12 March at 2301, associated with a small ash emission. Five COSPEC measurements yielded an average SO2 flux of 1,540 t/d, similar to the previous month. Deformation measurements showed no significant changes.

Seismic energy release and the number of earthquakes were at low to moderate levels in April. Seismicity peaked on 11 April with 162 low-frequency events. Earthquakes were dispersed around the crater, with focal depths of 0.5-6.5 km. Pulses of low-energy tremor began 26 April and persisted until the 29th, when an episode of continuous low-energy tremor was associated with a small ash emission. Dry and electronic tilt showed no substantial changes. Measurements of glacial behavior showed significant ablation, reaching a rate of the order of 240 m3/day. The average rate of SO2 emission, measured by COSPEC, was 1,467 t/d.

Seismic activity was at low levels during May and June, with a decrease in seismic energy in early June, then an increase to moderate levels during the second half of the month. Earthquake locations were aligned with the Palestina fault at depths of 0.5 to 12 km, or were oriented oblique to the fault and at shallower depths. Pulses of low-energy tremor were also recorded. There was no significant deformation, although there was a slight increase at the Refugio Station (7 km W of the crater). Glacial measurements indicated a decrease in ablation. June SO2 flux was relatively high, averaging 5,985 t/d as determined by COSPEC.

Ash emissions were reported during May, July, and August, frequently with associated tremor. Tremor intensity and the number of high- and low-frequency earthquakes fluctuated, roughly corresponding with increased explosive activity. SO2 flux was moderate with the exception of high fluxes recorded during June, and the geochemistry of the hydrothermal system remained unchanged from December 1988.

Ash emissions. Ash was emitted on seven days during May (5, 6, 8, 12, 14, 17, and 24), deposited primarily to the N, NW, and W. On 14 May, ash was reported falling in Manizales (25 km W of the volcano) from 1120 to 1530 with an average cumulative thickness of 0.43 g/m2. No ash was reported during June, but five small ash emissions were reported during July, with a total accumulation of 500 g/m2 at Refugio (2.0 km NW of the crater). The largest emission, on 25 July, deposited ash 30 km from the volcano. The deposits, 2 mm thick at Refugio, were composed exclusively of lithic material. Several small ash emissions were also reported during August.

Volcanic tremor. Tremor was frequently associated with the ash emissions (and with long-period events in August), and its frequency of occurrence and intensity roughly corresponded with the frequency and intensity of ash emissions. During May and July, tremor was more frequent and more intense; the reduced displacement was 3.2 cm2 on 12 May, 2.85 cm2 on 14 May, 3.05 cm2 on 22 May, and reached 3.5 cm2 in July (these were considered moderate to below moderate levels for Ruiz). During June and August when there was little or no ash emission activity, reduced displacements were 0.32 cm2 on 9 June, 0.88 cm2 on 26 June, and <1.0 cm2 in August. During May-August, tremor episodes had periods of 0.1-0.5 seconds (0.15-0.4 seconds during August) and originated at <1 km depth. These episodes usually occurred in pulses with durations of 5-15 minutes, but occasionally were continuous (13-15 May and the end of August).

A second type of tremor, characterized as low-intensity and short-duration, has been noted at Ruiz. It is considered to be from a deeper source and is not associated with ash emissions. During July it originated W of the crater at >=1 km depth.

Other seismicity. Seismic activity increased during May with 1,326 high-frequency and 1,982 low-frequency recorded earthquakes (up from 579 and 1,580 respectively in April). Of these, 247 high- and 156 low-frequency earthquakes, occurred during a swarm on 31 May. The earthquakes were centered in three clusters; NE of, SW of, and under the crater, aligned with the Palestina fault, and at depths from 0.5 to 4.5 km. During June, 1,664 high- and 1,422 low-frequency earthquakes of M <= 2.8 were recorded, at depths of 0.5-12 km. Epicenters were aligned with and transverse to the Palestina Fault. Seismicity increased during July, with mainly long-period earthquakes and bursts of seismicity consisting of many high- and low-frequency earthquakes occurring in trains. The earthquakes were located around the crater at depths of 0.5-5.0 km. A swarm of 400 earthquakes (M <=2.6), with three felt shocks at the onset, were recorded over a 6-hour period on 28 August (figure 37). The swarm was located NE of the crater with focal depths < 1.5 km (figure 38).

Figure 38. Epicenters of high-frequency earthquakes at Ruiz, August 1990. Courtesy of INGEOMINAS.

Deformation. Deformation continued to be monitored at Ruiz using a network of dry-tilt and EDM stations; one new EDM station was added to the network in June near the Bis seismic station (at 5,030 m, almost 6 km NW of the crater). A few short-term episodes of moderate deformation were noted during May, but no significant deformation was detected during June-mid-August. On 13 August, the Refugio EDM station showed a rapid 100-µrad displacement (figure 39), but this did not correspond with any other recognized changes in activity (seismic, geochemical, or deformational) at the volcano.

Measurement of topographic controls on the summit glaciers indicated a decrease in ablation in June with respect to April and May.

Plume geochemistry. COSPEC monitoring indicated a decrease in SO2 flux following high levels in June. The monthly average SO2 flux was 1,519 t/d in May (11 measurements with a range of 367-3,869 t/d), 5,985 t/d in June (three measurements, 5,208-7,498 t/d), 1,097 t/d in July (six measurements, 406-2,672 t/d), and 2,437 t/d in August (figure 40).

Figure 40. Monthly average SO2 flux from Ruiz, 1988-90. Upper line is calculated with measured wind data; lower line without wind data. Courtesy of INGEOMINAS.

Geochemistry of the hydrothermal system. The following is by S. Williams, S. Schaefer, and José Vasquez.

"The large white gas column continues to boil continuously from Ruiz. We visited and sampled hydrothermal springs between 11 and 16 August. The geochemistry of all of the sites has been studied and is reported in Sturchio and others (1988) and Williams and others (1990). His isotopic data are reported in Sano and others, 1990. The Azufrera Nereidas gas vent, located in the Nereidas valley at 3,575 m elevation, had maximum temperatures of 86°C. Gas release appeared to be unchanged from the previous visit in December 1988. The hot spring, located in the river canyon below the gas vent, was found to have temperatures of 51°C and pH of 6.1. The flow rate was estimated to be between 1 and 5 liters/minute. Abundant sulfur deposition is evident at the spring. In Botero Londoño hotspring, at 15 km distance and 2,450 m elevation, spouting hot water had a maximum temperature of 95°C and pH of 7.6. The springs closest to the crater, Río Gualí (at 3 km and 4,670 m) had a maximum temperature of 57°C and pH of ~3.8. These are located within ~50 m of the tongue of the glacier in the Gualí valley. Aguas Calientes, at 7 km and 3,780 m elevation, was found to be unchanged from the previous visit (December 1989) with a maximum temperature of 61°C and pH of 1.6. Other hot springs sampled in July included El Recodo and the Hotel Termales del Ruiz. El Recodo, a bicarbonate spring, had a temperature of 60°C and pH of 7.8, unchanged from measurements made in December 1986. The Hotel is a strong acid-sulfate-chloride spring with a temperature of 64°C and pH of 1.1."

The number of high-frequency earthquakes and total seismic energy release increased during September (figure 41). A swarm of high-frequency earthquakes on 17 September was centered NW of the summit at depths of 3.0-7.0 km (figure 42). Pulses of volcanic tremor (periods of 0.2-0.3 seconds) were recorded throughout the month, and were occasionally associated with small ash emissions. Tremor was continuous from 27 September through the end of the month, with associated ash emissions. Although dry-tilt did not show any significant changes, 6.56 µrad of deformation were measured 7 km from the summit (at Refugio station) on 4 and 8 September, with coincident seismic activity. The average SO2 flux for the month, measured by COSPEC, was 2,448 t/d. A "Japanese box" sampling device (an open beaker containing a 10 N KOH solution placed within the volcanic plume), operated 2.5 km from the summit in July, contained 1.52 mole % SO2 and 0.59 mole % HCl.

Many small ash emissions occurred during October, although seismicity remained at low levels. Two small swarms of high-frequency earthquakes were recorded on 14 and 22 October. Tremor episodes (2 cm2 maximum reduced displacement) were prominent and were occasionally associated with small ash emissions. Although EDM measurements showed important changes, dry-tilt did not show ground deformation. Similarly, EDM indicated 6.6 µrad of deformation at one station during September, while dry-tilt did not show any significant changes. The average SO2 flux for the month, measured by COSPEC, was 1,630 t/d, compared to 2,448 t/d in September.

Many small ash emissions occurred and high-frequency seismicity was at high levels during November. Hypocenters were located around the crater at shallow depths. Pulses of tremor occurred frequently, often associated with the ash emissions. Low-frequency seismicity was at low levels and there was no measured ground deformation. The SO2 flux continued to decrease, averaging 860 t/d.

Several ash emissions occurred during December. Seismic activity was characterized by high-frequency earthquakes (figure 43), centered W and SW of the the crater. Tremor was frequent, but of low energy. Ground deformation measurements showed no significant changes. The month's average SO2 flux was 1,464 t/d, up from 860 t/d in November.

Tremor episodes were brief (around 10 minutes) and of low intensity during the month. Low-frequency events were uncommon and of low energy. On 16 January at 0242, a M 3.8 high-frequency event, centered 1.5 km S of Arenas crater at 3.8 km depth, was felt around the volcanic complex.

Dry-tilt stations and the electronic tiltmeters at Refugio and Recio recorded no deformation. Morphologic changes were observed at Diablos Rojos B glacier. The average rate of SO2 emission measured during the month was about 1,140 t/d, down slightly from 1,464 t/d in December.

Sporadic, small ash emissions continued, while seismicity remained at low levels during February. No significant variations in energy release or number of events were recorded. High-frequency earthquakes occurred in three zones; at the crater, and to its S and E (earthquakes had not recently occurred in the latter zone). Low-level tremor pulses were often associated with the ash emissions. No significant changes in deformation were observed. The monthly average SO2 flux, measured by COSPEC, was 753 t/d.

Two high-frequency earthquake swarms were recorded in March, the first centered N of Arenas crater, and the second to the S. The number of events, energy released, and tremor amplitude were all at low levels. No significant changes in deformation were observed. Ashfall was reported up to 30 km WNW (Villamaría), from an emission on 5 March. The monthly average SO2 flux, measured by COSPEC, was 2,233 t/d, an increase from 753 t/d in February and 1,140 t/d in January.

An increase in the number of tremor pulses preceded several days of ash emission at the end of April. Lithic and crystalline ash (<2 mm in diameter) was reported W of the volcano in Pereira (40 km from the summit), Santa Rosa de Cabal (35 km), Chinchiná (35 km), and Manizales (25 km), and NE of the volcano in Mariquita (55 km). High- and low-frequency seismicity was generally at low levels in April, with a slight increase in released energy from low-frequency events. The monthly average SO2 flux, measured by COSPEC, was ~2,740 t/d, up from 2,233 t/d in March.

Lithic ash emissions were frequent during May, depositing material to Manizales (30 km WNW) on 1 May. Short pulses of shallow tremor were associated with the emissions. High-frequency seismicity reached very high levels during a swarm on 8 May (figure 45), which included a M 2.1 earthquake, 2.5 km N of Arenas crater at 5 km depth. A similar swarm occurred on 14 May. Low-frequency seismicity was at a moderate level in May, with peaks of vigorous seismicity on 4 days. Deformation measurements showed no significant changes. The SO2 flux was low; the monthly average was 930 t/d, compared to ~2,740 t/d in April.

Activity was generally at low levels in June, although there were one large ash emission and a high-frequency seismic swarm. The ash emission was associated with low to moderate levels of tremor, and deposited material on Manizales (30 km WNW). One earthquake, located 2.5 km S of the summit crater, was felt during the swarm. The monthly average SO2 flux, measured by COSPEC, was 2,275 t/d, compared to 930 t/d in May and ~2,740 t/d in April. Deformation measurements did not show significant changes.

Seismicity was at very low levels in July, although tremor reached slightly higher levels at the beginning of the month. Deformation measurements showed no significant changes. The SO2 flux continued to fluctuate, with a monthly average of ~1,220 t/d. Two small ash emissions, restricted to the summit region, were observed during July.

Seismicity remained at low levels in August, with earthquakes mainly W and N of the crater at 0-5 km depths. Tremor episodes were brief and of low energy. Deformation showed no significant changes. The monthly average SO2 flux was 1,135 t/d, similar to July.

The number of seismic events and the amount of released energy were both at low levels in September. Only a few short pulses of very low-energy tremor were recorded. Deformation showed no significant changes. The monthly average SO2 flux was low, declining to ~925 t/d from 1,135 t/d in August.

Although seismicity remained at very low levels in October (figure 46), there was a slight relative increase in long-period activity. High-frequency earthquakes were centered NE and SW of the summit. The SO2 flux varied between moderate and low values, and deformation measurements showed no significant changes.

Weak steam emission from the active Arenas crater produced a white plume 800 m high in January. Volcanic seismicity was at very low levels and tremor was infrequent. A small swarm of high-frequency earthquakes on 2 January caused a peak in seismicity, with high levels of released energy. High-frequency events were centered around Arenas crater and the S flank at 0.5-5 km depths. Deformation measurements showed no significant changes.

A high-frequency earthquake swarm in mid-March and early April ended nearly two years of low activity. Significant long-period earthquakes began in mid-April. Several swarms on 19, 22, and 23 April culminated in an explosion at 1554 on the 23rd. Seismic activity gradually declined after the explosion. The Emergency Committee of Caldas declared a yellow alert and suspended visitor and tourist passes until the seismicity had decreased to acceptable levels. [INGEOMINAS stated that there was no emission of ash at the time of the 23 April earthquake swarm.]

Almost two years of low-level seismicity ended in mid-March 1994 with the occurrence of a high-frequency earthquake swarm followed by long-period events and an explosion on 23 April (BGVN 19:05). Activity returned to low levels through the rest of 1994.

A mid-sized landslide in January 1995 descended the upper reach of the Lagunillas River but caused no significant damage. It was primarily caused by ground and ice-cap instability, not volcanism. Seismicity in July and August 1995 was stronger than in April 1994. Swarms of long-period events reached a maximum count of 1,050 events on 26 July with more than 6.3 x 108 ergs of energy released. Some of the events were related to explosions heard by scientists doing fieldwork some kilometers away from the Arenas Crater, but ash emission was not confirmed. No significant volcano-tectonic activity was registered. Swarms of long-period events during early September 1995 were similar to those of July-August, but were fewer in number and had less energy. This volcanic related seismicity was located mostly toward the Arenas Crater and the SW part of the volcano at shallow depths.

Seismicity during January-April 1996 remained low, except for the first 10 days of January when there was an increase of long-period screw-type events, with a high of seven on the 5th. Most of these events were located at shallow depths near Arenas Crater and over its W side. Screw-type events have become significant since May 1995. Some volcano-tectonic earthquake swarms also occurred during these four months. Two significant swarms were located toward the S part of the volcano, near the RECI seismic station (figure 47). In both swarms, maximum magnitudes were close to 3. Tremor signals were intermittent; some saturated the stations closest to Arenas Crater, but none were correlated to ash emissions. The electronic tiltmeter 800 m from Arenas Crater (FARA) did not show significant variations. During these four months there were a total of 657 volcano-tectonic earthquakes and 1,308 long-period events recorded by the observatory network. This suggests that processes related to fluids within the volcanic conduits were dominant over fracture-related processes.

Figure 47. Location of telemetered stations and significant seismic events recorded at Ruiz during January-April 1996. Courtesy of INGEOMINAS.

Nevado del Ruiz, located 33 km SE of Manizales, is a broad stratovolcano of andesitic and dacitic lavas and andesitic pyroclastic deposits that cover more than 200 km2. Steep headwalls of massive landslides cut its flanks, and melting of its summit ice cap during historical eruptions resulted in devastating lahars. The last eruption began with moderate phreatic ejections on 11 September 1985. On 13 November 1985 an explosive eruption produced pyroclastic flows and surges that melted part of the summit ice cap. Major mudflows subsequently devastated Armero and other towns on the flanks of the volcano, causing over 23,000 fatalities. Intermittent minor ash emissions with occasional stronger phreato-magmatic eruptions continued until July 1991.

During May-July, seismic activity at Ruiz remained quite low. Significant volcano-tectonic earthquake swarms occurred on 8, 10, 11, 16, and 23 May, and 7, 15, and 18 June (figure 48). Most were located at depths of <7 km and within 3 km of Arenas Crater. The strongest volcano-tectonic earthquake (M 2.2) was recorded at 1636 on 10 May. Swarms of long-period events were registered on 9, 20, 23, and 25 May. Scientists working in the field reported that an isolated long-period event at 1153 on 29 May was correlated with an explosion-like sound possibly caused by the fall of solid material. The analog recorders detected this event, but the digital systems did not.

Figure 48. Released energy and number of volcano-tectonic and long-period events at Ruiz during May-July 1996. Scales are approximate. Courtesy of INGEOMINAS.

Visual monitoring indicated that normal white gas plumes occurred over the Ruiz summit and reached an altitude of <2 km. The FARALLONES electronic tiltmeter did not record any significant deformations during May-July .

A new fumarolic field and a hot spring, both called "El Calvario," were found 1.7 km NE of Arenas Crater at an elevation of 4,628 m. The fumarole had a temperature of 84°C and pH of 3.8. Emissions consisted of: H2O vapor, 95.5%; CO2, 4.3%; total S, 0.18%; and HCl, 0.001%. The water from the hot spring had the following features: temperature, 66.4°C; pH, 2.7; Cl, 10 ppm; and SO4, 1,545 ppm.

A press release on 31 March noted that small long-period earthquakes had been detected at Ruiz throughout the month, although some may have been related to glacier movement; one long-period event on 24 March saturated the seismic stations near the crater. After several months of low seismicity, a moderate swarm of 80 volcanic-tectonic earthquakes within an hour was recorded on 15 April. The largest had a magnitude of 1.3. Small long-period earthquakes were present during the entire month of April, centered on the SW flank near the crater. Seismicity was still at low levels as of 25 May.

The last reported activity at Nevado del Ruiz was a moderate earthquake swarm during late March through April 1999 (BGVN 24:04). On 9 June 2002 at 2300 another swarm of volcano-tectonic earthquakes began. Following the swarm, hundreds of hybrid earthquakes were recorded, with more than 1,300 earthquakes occurring in 16 hours. High seismicity marked the following 3 days during which a total of ~2,300 earthquakes were recorded. This is the highest number of events recorded per day at Nevado del Ruiz since 1985. According to news reports, the earthquakes had magnitudes up to ~2.3 and occurred at depths of 0.5-3 km. In addition to heightened seismicity that was felt by people near the volcano, jet-like sounds corresponded with some of the hybrid earthquakes, and a strong odor of SO2 was reported near the summit. No ash emissions were reported, and seismicity decreased by 13 June. At the height of the activity the Alert Level was at Orange.

Our last report on Nevado del Ruiz (BGVN 27:05) focused on a swarm of earthquakes that occurred in June 2002, when the Instituto Colombiano de Geología y Minería (INGEOMINAS) raised the Alert Level to II (“Orange”, on a scale from I-IV, where the highest Alert Level is I, “Red”). Monthly reports from the INGEOMINAS Manizales Observatory became available online beginning in March 2006 and continued through the time period covered in this report, March-December 2006. We also include the long-term datasets of deformation recorded from 1988 to 2006 and radon-gas monitoring from 2002 to 2006.

INGEOMINAS characterized overall activity at Nevado del Ruiz from March to December 2006 as limited to small earthquakes, minor rockfalls, and intermittent vapor plumes. They measured continuous deformation trends (primarily from 3 tilt stations), and low levels of radon-gas emissions with a peak in March 2006 at two stations. No Alert Level was defined for this time period.

Seismicity during 2006. INGEOMINAS reported that low seismicity generally prevailed from March through December 2006; 217-673 events occurred per month at depths of 6-10 km below the summit with maximum local magnitudes of 0.95-2.3. Long period (LP) events occurred slightly more often than volcano-tectonic (VT) events; 1-5 hybrid events were detected each month except in December when these events were absent. Tremor was recorded only once in September and twice in October.

Two small VT earthquake swarms were recorded, one in March and the other in May. The swarm on 9 March occurred as a cluster of events 2-4 km deep centered to the SE of the crater (figure 49). The swarm on 29 May was characterized by ~20 events located SE of the crater with magnitudes less than 1.02.

Figure 49. Maps and cross-sections showing epicenters and hypocenters for VT events detected at Nevado del Ruiz for March (top) and December (bottom) 2006. Note that the colored depth scales are different for the two months (labeled as “Profund.” in each frame). The bottom and right-hand edges of the maps show vertical cross sections with horizontal lines at 2 km depth intervals. More earthquakes were located in March (647) than December 2006 (290), however, events as deep as 8 km were located in December. Courtesy of INGEOMINAS.

Rockfalls, ice movement, and debris flows were also detected by the seismic network from March to December 2006. For these kinds of events, typically more than 250-600 per month were detected; however, fewer events were detected in April and June, 61 and 47 events respectively.

Vapor plumes in 2006. During most of 2006 vapor plumes were visible from the summit area of Nevado del Ruiz. Often appearing intermittently, these plumes were white or white-to-gray colored and reached 100-600 m over the crater rim. Plume emissions have been associated with the fumaroles within the summit crater.

Deformation summary. The deformation network at Nevado del Ruiz in March 2006 contained 12 dry tiltmeter stations (Piraña, Rubí, Bis, Molinos, Tumbas, Refugio, Nereidas, Pequeño, Pijao, Arenales, Alfombrales, and Recio) and four stations for leveling campaigns (Piraña, Bis, Tumbas, and Nereidas; figure 50). This network was developed to cover the W and N flanks; the S and E flanks did not have network coverage during this reporting period. Fieldwork was planned to include leveling at sites Arenales, Alfombrales, and Recio since they had not been occupied for several years. Four tilt stations showed inflation and deflation trends (Bis, Nereidas, Refugio, and Tumbas) and are discussed in the text below.

Tilt station Bis was established in late 1988 on the NW flank ~5.7 km NW of the active crater (figure 50). As seen on figure 51, inflation had been recorded at Bis from 1988 through 1999 with a cumulative tilt increase of as much as 40-60 microradians (µrad; figure 51). Since the beginning of 2000 until the end of 2004, this station recorded stable conditions with a small amount (4 µrad) of inflation. From 2004 through March 2006 there was another significant increase in the inflation trend; a cumulative 22 µrad N component and 15 µrad E component inflation.

Figure 51. Deformation trends measured by the dry tilt station “Bis” (on the NW flank of Nevado del Ruiz) from 1988 to March 2006. N and E components are shown in blue and red, respectively. Courtesy of INGEOMINAS.

Tilt station Nereidas was installed 4.6 km SW of the active crater and measured significant changes primarily from late March 1993 to March 2006. Inflation and deflation trends were recorded, 13 µrad N and 11 µrad E, respectively.

Station Refugio, located 2.6 km NW of the active crater, has primarily recorded stable conditions since 1990.

Station Tumbas was located on the NW flank of the volcano ~4.8 km from the crater. This station has shown deflation from both components since 2000. Since February 2005 the cumulative deflation of the N component was 7 µrad and 13 µrad in the E component. From 2005 to March 2006 there were fluctuations from this station within the measurable range of the tiltmeters.

Long-term radon gas measurements. Radon monitoring at Nevado del Ruiz has been based on six stations. In particular, INGEOMINAS has long records from stations Gualí and Hotel Termales since 2002 and 2003, respectively (figure 52). The locations of the radon gas sampling stations were not disclosed, however a 1986 map of instrumentation places the Hotel Termales (labeled “Termales” with a square and “X”)and Río Gualí (marked with a circle near the river) stations within 12 km to the NW of the summit crater (figure 53). In March 2006, results from three radon monitoring sites suggested to INGEOMINAS that there was a possible correlation with the earthquake swarms detected on 9 March 2006. Stations Río Gualí, Gualí, and Rubí recorded an increase in radon emission on 5 March while stations Condor and Cajones maintained low levels (55 pico Curies per Liter, pCi/L). From April through December 2006, no major changes were noted in radon gas emissions.

Our last report on Nevado del Ruiz (BGVN 37:07) summarized monitoring efforts by the Instituto Colombiano de Geología y Minería (INGEOMINAS) volcano observatory based in Manizales, highlighting the long records of geophysical and radon-gas data starting in 1988 and continuing through 2006. Here we follow up on volcanic activity from 2007 to 2012, including an escalation leading to explosions in February 2012. Elevated seismicity, wide-spread ashfall, and very high SO2 fluxes (~30,000 tons/day) resulted in a Level I Red Alert announcement (on a scale from IV to I, Alert Level I is the highest, “Red Alert”) in June 2012 and public notices of evacuations. Activity subsided in July 2012 and remained low through the remainder of this reporting period ending 9 September 2012.

Figure 54. Maps of located earthquakes at Nevado del Ruiz during the month of April 2010. (Left) This map shows the distribution of VT events and cross-sections for depths in 1 km intervals; the 15 April 2010 swarm is circled. (Right) This map shows 209 registered LP events (M 0.09-2.15); frequencies were below 5 Hz with average event durations of 0.3 s. LP events were concentrated in a zone to the W of the crater, a characteristic observed in records since 2006. Courtesy of INGEOMINAS.

Table 3. Seismicity types and counts at Nevado del Ruiz registered from 2006 to September 2012 compiled from INGEOMINAS reports. The LP Total column accounts for all forms of LPs including hybrid and tornillo when present; tornillo earthquakes are described by Narváez and others (1997). The TR/TO column contains tremor (“TR”) and tornillos (“TO”). Epicenter Clustering refers to directions relative to the crater, and to epicenters occurring within the immediate crater region “C”. Notable Seismicity includes swarms with dates and the number of events provided when known in parentheses; seismicity interpreted as possible explosions is listed as “ES” (explosion signature); multi-events (“ME”) refer to seismicity that is described in figure 56; pseudo-tornillo events are listed (“PT”), a class of earthquakes also detected at Galeras volcano (BGVN 37:04) and illustrated in figure 55. For all entries with “na,” this represents seismicity that has been recorded but only tallied within the LP Total column. The “x” indicates values not currently available. Shading (yellow, orange, and red) corresponds to the alert announcements released by authorities according to the level of hazardous conditions. Courtesy of INGEOMINAS.

Geodesy, 2007-August 2010. Deformation monitoring expanded in late 2007 when INGEOMINAS installed additional electronic tilt stations, augmenting their dry tilt datasets. Dry tilt measurements had been recorded since at least 1986 (see the station distribution map, figure 10 in BGVN 37:07). While the term “dry tilt” is pervasive in volcano monitoring literature, this can cause confusion as it was originally adopted to differentiate measurements made with water leveling techniques (Yamashita, 1992). Alternative terms are “single-setup leveling” or “tilt leveling” however, the term “inclinómetro seco,” has been used consistently throughout INGEOMINAS monthly technical reports since March 2006. Tilt measurements collected with site occupation techniques are manually intensive, requiring extensive field time, reliable benchmark pairs, a spirit level, and leveling rods. In August 2010, dry tilt values were available from three stations and electronic tilt values were available from five operating stations; results were reported in the INGEOMINAS technical bulletin (available online).

In August 2008, electronic distance meter (EDM) base stations and reflectors were installed on the W flank of the volcano. Site occupations at Olleta and Refugio recorded stable conditions from September 2008 through August 2010.

Gas emissions, 2007-August 2010. Frequent steam plumes were visible reaching 50-850 m above the crater from January 2007 through August 2010. On 17 July 2010, the Washington Volcanic Ash Advisory Center (VAAC) was alerted to a spike in seismicity detected at Nevado del Ruiz. Several aviation alerts were released; however, no volcanic ash was detected in satellite imagery and advisories were canceled that same day. Several peaks in diffuse soil CO2 emissions were detected in mid-2008 from two geochemical stations, Gualí and Cajones (N and S of the summit, respectively).

Radon-gas emissions measured at Gualí and Cajones also showed peaks in early 2010. INGEOMINAS had maintained emission records since 1995 and was investigating links between radon emissions and earthquakes (Garzón and others, 2003). Radon hazard investigations had been conducted in Manizales (located ~30 km NW of the volcano) by INGEOMINAS that determined water supply and household levels of radon (Salazar and others, 2003). This baseline data was mapped for SE Manizales and showed low levels of radon in water supplies and also low levels at the 43 indoor sites where passive sampling detected an average of 1.9 pCi/L.

During fieldwork on 30 November-1 December 2009, INGEOMINAS installed two scanning Differential Optical Absorption Spectrometer (DOAS) systems within 5 km W of the edifice. Stations Bruma and Alfombrales were telemetered to send SO2 flux data to the Manizales observatory where results were analyzed with NOVAC software. The Network for Observation of Volcanic and Atmospheric Change (NOVAC), designed by the European Commission’s Sixth Framework Program, supported this installation. Colombia was one of seven countries participating in the program that sought to monitor and assess SO2 emissions from active volcanoes (Galle and others, 2009). During 2-29 December, SO2 flux ranged 195-554 t/d at Bruma and 41-140 t/d at Alfombrales.

Escalating seismicity from September 2010 to 2011. Seismicity notably increased in September 2010 and prompted authorities to raise the alert to Level III (Yellow, on the four-level scale) on 30 September (table 3). Within four months, pseudo-tornillo earthquakes (figure 55) and possible explosive signatures appeared in the seismic record. From September 2010 through December 2011, an average of more than 890 VT earthquakes per month were recorded, almost eight times as many events as recorded during the previous 12 months. A similar increase in LP events was also observed during this time period; however, epicenters were clustered in the same regions as previous years: within the crater, to the SE, S, SW, and W (as in figure 54).

Figure 55. This long-period earthquake (described as a pseudo-tornillo) was recorded on 6 January 2011 at 1343 from Nevado del Ruiz on seven seismic stations (appearing strongest on station BISz, the trace second from the top). BISz is the closest seismic station to the volcano, located ~2 km W of the crater. The spectra (right) show a dominant frequency of ~6.25 Hz; this characteristic, in addition to the relatively short coda, classified the event as a pseudo-tornillo (Narváez and others, 1997). Courtesy of INGEOMINAS.

A type of earthquake classified as “multi-event” began to appear in February 2011 (see ME events in table 3). These events frequently occurred from February through August and were attributed to small explosions and degassing (figure 56). Tremor and tornillo earthquakes were recorded in March of 2011 and, over the next six months, occurred more frequently with time.

Figure 56. Seismic traces of a “multi-event” registered at 1351 on 6 October 2011 as recorded at five stations around Nevado del Ruiz. The earthquake appeared strongest at BISz, the closest station to the volcano, and much weaker-to-unrecognizable at other stations. Courtesy of INGEOMINAS.

Geodesy, September 2010-2011. During September 2010-2011, INGEOMINAS recorded stable conditions with minor fluctuations from the EDM stations Refugio and Olleta. Both stations were surveyed in February, October, and November 2011, and only Refugio was surveyed in September and December.

INGEOMINAS noted an increasing trend at the electronic tilt station LISA that began in October 2010 and continued through 2011; the two components registered a cumulative increase of 20 µrad. RECIO had been recording stable conditions until May 2011; from May through December 2011, the N component increased by 23 µrad and the E component decreased by 10 µrad. Corrective measures had been taken to protect the BIS and REFUGIO tilt stations from thermal effects, however, cyclical changes persisted in their datasets. By December 2011, seven electronic tilt stations were online and were recording minor fluctuations primarily due to temperature change.

Permanent GPS stations Gualí and Nereidas were installed on the lower W flanks between May and August 2011 and a third station, Olletas, was online by November 2011. GPS instrumentation and continuous data processing were part of a collaborative effort between INGEOMINAS and the University of Wisconsin, Madison.

SO2 emissions, 2010-2012. Since installation of the two scanning DOAS stations in late 2009, background levels of SO2 were rarely higher than 1,000 t/d until September 2010. INGEOMINAS recorded increased SO2 emissions in late 2010 (figure 57), while plumes rose to heights of 220-1,000 m above the crater (averaging ~700 m) through 2011. An increase was observed from November 2010 through much of 2011; maximum daily values of SO2 flux frequently exceeded 1,500 t/d. Occasional peaks above 3,000 t/d were recorded from November 2010 to January 2011 (a), June-July 2011 (b), and November 2011 to February 2012 (c). Beginning in February 2012, emissions dramatically increased during a period of escalated seismicity (table 3). SO2 flux peaked during May and June; the three strongest peaks were greater than 33,000 t/d. By late June, emissions were declining.

Explosive activity in 2012. In late January 2012, while SO2 flux began to increase dramatically (figure 57), explosion signatures (also described as strong degassing events) and multi-events continued to appear in the seismic records. On 8 March an overflight of the summit provided INGEOMINAS scientists a view of ash-covered snow on the E flank and near the crater rim (figure 58); in their monthly report, INGEOMINAS suggested this ash may have fallen during an explosion detected on 22 February 2012.

Figure 58. This photo was taken during a flight past Nevado del Ruiz’s active crater at 0705 on 8 March 2012. Viewed from the Azufrado sector (NE of the summit crater), a column of gas was rising to a maximum height of ~1,400 m above the crater. A thin layer of ash was visible on the snow near the crater (in the foreground of the image). Courtesy of INGEOMINAS.

Based in part on information captured by webcameras around the volcano (including one in Manizales located 30 km NW of Nevado del Ruiz), INGEOMINAS reported that plume heights had increased significantly in March 2012 (figure 59). Reports from local populations around the volcano also alerted INGEOMINAS of sulfur odors. Residents smelled these odors during March; April, May, and August reports were from Manizales, Lebanon, Palocabildo, and Chinchiná.

Figure 59. (Top) The map of Nevado del Ruiz’s geophysical monitoring network includes webcameras, meteorological stations, mudflow stations with acoustic flow sensors, and infrasound. (Bottom) Plot of plume height above the crater as measured from webcameras located near the flanks (including sites Piraña (PIRA), Gualí (GUAL), and Manizales (OVSM)) from January through June 2012. Courtesy of INGEOMINAS.

The national park surrounding the volcano, Los Nevados National Park, closed in April 2012 due to possible ashfall and lahar hazards. The rainy season (March-June) had begun and mass wasting on the steep slopes, especially of remobilized ash, was a major concern. “Most lahars are initiated as dilute, subcritical flows high on volcanic slopes, but quickly increase their volumes as they incorporate sediment along travel paths (Lockwood and Hazlett, 2010).”

On 16 and 19 April 2012, INGEOMINAS observed ash emissions from the summit and on 22 April, Washington VAAC announced possible ash in the steam plume. Volcanic ash was detected later with satellite imagery, spreading ~110 km NE of the summit on 29 May.

Seismicity decreased in early May 2012 to levels observed before the escalation began in February, and fewer explosions and multi-events were recorded. On 3 May authorities lowered the alert to Level III (Yellow). Conditions at Nevado del Ruiz continued to change, however, and when seismicity abruptly increased, the Alert Level was raised to Level II (Orange) on 29 May (table 3, figure 60). That day, explosions from the crater generated ash plumes that dispersed over more than 20 communities located to the WNW, NW, and NNW. Washington VAAC released four notices on 29 May describing ash up to 11 km altitude. News media reported that three primary airports in the region (Manizales, Pereira, and Armenian) collectively canceled ~20 flights that affected ~700 passengers on 29 May.

Figure 60. A seismic record from Nevado del Ruiz starting just prior to 29 May 2012 and ending slightly past noon on 1 June 2012. The notes explain the start of ash emissions (top shaded bar), alert announcement (orange diamond), and intervals of tremor (shaded bars with orange connected lines). Translation of text: Initial pulse of ash emission at 0397 on 29 May. Throughout the seismogram, volcanic tremor is present and in parts, appears as banded tremor that increases in amplitude. Courtesy of INGEOMINAS.

Widespread ashfall in early June 2012 required field maintenance by INGEOMINAS to clear ash from solar panels and equipment (figure 61). Imagery captured by the NASA satellite EO-1 revealed a two-toned summit disclosing partial ash cover over the white summit glacier (figure 62). The seismic station INDERENA, acoustic flow station MOLINOS, and the radio repeater that served Nevado del Ruiz, Tolima, and Santa Izabel volcanoes were disabled due to ash cover. Washington VAAC released advisories regularly until 24 June; ash reached altitudes in the range of ~5.5-7.6 km. Plumes tended to drift N, NW, WNW, and W; however, an ash plume on 8 June drifted ~28 km SE. The range of plume lengths was 28-110 km until a period of quiescence during 25 June-2 July.

Figure 61. Ash covered several solar panels as well as field equipment located near Nevado del Ruiz’s W flank in June 2012. Here, at near-equatorial latitude (~5° N), the panels are typically oriented near-horizontal for effective solar exposure which also makes it easy for ash to collect and not wash away. Courtesy of INGEOMINAS.

Figure 62. (Left) This image was taken by the NASA Expedition 23 crew on 23 April 2010, with a Nikon D3S digital camera fitted with an 800 mm lens. A steam plume drifts SW from the summit crater, blending in with the snow-cover. The summit crater is indicated with a black arrow and the neighboring features, Cráter de Olleta and Altas de Piraña correspond with the outlined field of view in yellow in the left image. Note the scale is approximate and there is some skew to this image as it was taken from a shuttle flight as opposed to the orbiting satellite. Courtesy of NASA. (Right) This satellite image of Nevado del Ruiz was taken during significant ash explosions on 6 June 2012. The summit glacier displays the sharp contrast of muted gray on the NW due to ash cover and bright white on the SE where ash had not fallen. The black arrow points to the summit crater and white clouds are concentrated in the NW and SE corners of the image that also partially cover the peak Altas de Piraña. Image courtesy of NASA by Jesse Allen and Robert Simmon using EO-1 Advanced Land Imager data.

On 30 June 2012, seismicity increased and large plumes of ash vented from the summit (figure 63). At 1700 that day, authorities raised the alert to Level I (Red). Local news media reported the preventative evacuation notice provided by the Emergency Committee of Caldas; Caldas is the department of Colombia encompassing Nevado del Ruiz and six districts, 27 municipalities, and the capital, Manizales. An estimated 300 families were ordered to evacuate from the rural zones of districts Chinchiná (30 km WNW), Villamaría (28 km NW), Palestina (40 km WNW), and Manizales (30 km NW) due to both escalated explosions and also the potential for flooding along the rivers Chinchiná and Río Claro. In the Department of Tolima, located S of Caldas there was a recommendation to evacuate 1,500 families in risk zones in eight municipalities.

Figure 63. A snapshot of the seismic record from Nevado del Ruiz on 30 June 2012 and annotated to mark when officials announced the maximum Alert Level (Level I). Colored circles indicate events associated with fracturing (red), gas and fluid movement (yellow), and tremor resulting from gas or ash emissions (blue). Note that time stamps are not included except for the 1740 arrow. Courtesy of INGEOMINAS.

On 2 July 2012, Washington VAAC announced a 7.5-km-wide plume visible in satellite imagery that had drifted ~75 km W. Seismicity was decreasing, however, and that same day, authorities lowered the Alert Level to II (Orange). Airborne ash remained visible in satellite images until 8 July and continued to be observed at low elevations based on webcamera images. Ashfall was reported in Pereira (40 km WSW) on 11 July, and on 31 July a plume of ash and gas was observed rising 300 m above the crater.

Low levels of tremor had been detected in late July and throughout much of August 2012. Seismic swarms were detected on 12 and 13 August (table 3) with ~140 low-magnitude events under 5 km deep concentrated WSW of the Arenas Crater. On 6 August, ashfall was reported in Manizales and Chinchiná; on 12 August there were reports of ash in Manizales and Brisas (50 km SW). Through the end of August, plumes (ranging 200-800 m above the crater) were visible from the summit. Field measurements by INGEOMINAS and remote sensing with OMI determined that SO2 emissions remained high (figure 64) through August and early September. On 5 September 2012 authorities reduced the Alert Level to III (Yellow).

Figure 64. A Nevado del Ruiz SO2 plume was detected by the Ozone Monitoring Instrument (OMI) on NASA’s AURA satellite on 9 September 2012 from 1328-1507 (local time), extending well over the Pacific Ocean. The mass of SO2 was 1.28 kt, covering an area of 44,199 km2, and the maximum was 4.23 Dobson Units (DU) at 1331 local time. Courtesy of Simon Carn, Michigan Technological University and Joint Center for Earth Systems Technology, University of Maryland Baltimore County.

Recalling 1985 and additional hazard mitigation efforts. Nevado del Ruiz’s most deadly natural disaster was a lahar that, on 13 November 1985, scoured the Lagunillas River (E flank drainage system) and suddenly flooded the towns of Armero, Chinchiná, Mariquita, and Honda (figure 65). Armero was completely destroyed and more than 23,000 residents died. Light ashfall had been reported that day and a seismic network was in place, but no early warning system had been established to initiate evacuations (Lockwood and Hazlett, 2010).

Figure 65. Released in 2007, this hazard map of Nevado del Ruiz is dominated by lahar and pyroclastic flow scenarios. Highest risk areas are shaded red with lower risk areas in yellow; note that the town of Armero (Antiguo Armero, 48 km E of the summit) is in a region of high risk. A topographic assessment augmented with substantial field evidence determined flow paths and inundation probabilities within the major drainages of Gualí, Azufrado, Lagunillas, Recio, and Chinchiná (listed clockwise starting with the NE drainage). Pyroclastic flow, ashfall, and lava inundation were also considered and the radial sectors directed NE attribute hazards to lateral explosions based on crater morphology and geologic mapping of tephra units. Names highlighted in green indicate major towns. Courtesy of INGEOMINAS.

Since 1985, realtime geophysical monitoring greatly increased, including acoustic flow sensors designed to detect impulsive flooding in local drainages. Other advances included mobile gas monitoring (mini-DOAS) that augmented routine geochemical sampling at Nevado del Ruiz and recent hazard map revisions that emphasized inundation scenarios with zoning that clearly communicates areas at highest risk (figure 65). International collaborations with universities and agencies (for example, the University of Wisconsin and the European Union mentioned previously) have focused on mitigation efforts through training and technical resources.

Following the disastrous 1985 lahars, the USGS and the U.S. Office of Foreign Disaster Assistance (OFDA) developed the Volcano Disaster Assistance Program (VDAP) to respond to selected volcanic crises around the world (Ewert and others, 1997). The VDAP mission is to work with international counterparts to reduce fatalities and economic losses in those countries experiencing a volcano emergency. The VDAP website states that "Between crises, VDAP scientists focus on building and improving volcano monitoring systems and conduct joint activities to reduce volcanic risk by improving understanding of volcanic hazards [figure 66]."

Figure 66. The USGS/OFDA Volcano Disaster Assistance Program sent a team of scientists to aid INGEOMINAS and local authorities mitigating risk at Nevado del Ruiz on 28 May 2012. Courtesy of The Columbian.

During September 2012-July 2014 the Servicio Geológico Colombiano (SGC) maintained Alert Level Yellow at Nevado del Ruiz due to elevated SO2 flux, seismicity, and intermittent ash emissions; the Alert Level had been lowered to Yellow on 5 September. No major geophysical changes were noted during this time period.

Gas plumes were frequently visible rising as high as 3,500 m above the summit (figure 1). Minor gas-and-ash plumes occasionally occurred throughout this reporting period, and were often associated with episodes of tremor. Ashfall was reported from areas around the volcano (figure 2), notably on 11 July 2013 when ash reached Manizales (~30 km NW). The Washington Volcanic Ash Advisory Center (VAAC) reported ash plumes visible in satellite images on 10 October 2012 (reaching ~7 km altitude) and 11 July 2013 (reaching ~6 km altitude). A possible ash plume was detected at ~7.6 km altitude on 16 June 2013.

Figure 2. Persistent ashfall challenged the local monitoring systems for Nevado del Ruiz in September 2012. These two stations, a ScanDOAS station Bruma/Azufrado (left) and the GPS station at Gualí (right), were the focus of fieldwork by SGC staff in September. Courtesy of SGC.

SGC reported continuous and elevated SO2 emissions during this reporting period. Gas monitoring included satellite images, field installations (ScanDOAS), and MobileDOAS. ScanDOAS values captured the dramatic increase in SO2 flux during the explosive eruptions of early 2012 (BGVN 37:08). Since September 2012, the maximum daily average frequently exceeded 6,000 tons per day and exceeded 9,500 tons/day three times: twice in June 2013 and once in July 2014. MobileDOAS measurements were collected by the SGC during field campaigns and the values are reported in monthly technical bulletins available online (in Spanish).

Seismicity. Volcano-tectonic earthquakes (VT), long-period earthquakes (LP), and tremor were detected by the monitoring network throughout this reporting period. Earthquakes were detected from the summit glaciers, as well, notably in February 2014. Increased VT seismicity was notable during October 2012 (figure 3), 23-24 February 2013, 13 April-4 May 2013, 5-10 October 2013, 18-27 June 2014, and 29 July 2014 (figure 4). Maximum local magnitudes of VT earthquakes in the range of 1.6-4.4 were detected during this time period at depths of 0.5-13 km.

Figure 3. Seismicity from Nevado del Ruiz during October 2012 was elevated; 2,179 VT earthquakes were located. Two main areas of concentration were located N and NE of the summit (circled in red). Black squares are seismic stations; color coding refers to depth; circle size correlates with earthquake magnitudes. Courtesy of SGC.

Figure 4. Elevated seismicity from Nevado del Ruiz on 29 July 2014 consisted of 746 VT earthquakes detected in a zone ~2 km long SSW of the main crater, Arenas. Red lines correspond to local faults as well as Palestina, the regional, cross-cutting fault trending NE-SW. Depths of earthquakes are correlated to colors in the key as well as magnitudes based on circle size. Courtesy of SGC.

Volcano Types

Rock Types

Tectonic Setting

Subduction zoneContinental crust (> 25 km)

Population

Within 5 kmWithin 10 kmWithin 30 kmWithin 100 km

3,849
20,146
507,608
4,252,435

Geological Summary

Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers >200 sq km. Three major edifices, composed of andesitic and dacitic lavas and andesitic pyroclastics, have been constructed since the beginning of the Pleistocene. The modern cone consists of a broad cluster of lava domes built within the caldera of an older edifice. The 1-km-wide, 240-m-deep Arenas crater occupies the summit. The prominent La Olleta pyroclastic cone located on the SW flank may also have been active in historical time. Steep headwalls of massive landslides cut the flanks. Melting of its summit icecap during historical eruptions, which date back to the 16th century, has resulted in devastating lahars, including one in 1985 that was South America's deadliest eruption.

References

The following references have all been used during the compilation of data for this volcano, it is not a comprehensive bibliography.

This compilation of synonyms and subsidiary features may not be comprehensive. Features are organized into four major categories: Cones, Craters, Domes, and Thermal Features. Synonyms of features appear indented below the primary name. In some cases additional feature type, elevation, or location details are provided.

Synonyms

Paramo de Ruiz

Craters

Feature Name

Feature Type

Elevation

Latitude

Longitude

Arenas

Crater

5321 m

4° 53' 42" N

75° 19' 23" W

Domes

Feature Name

Feature Type

Elevation

Latitude

Longitude

Cisne, Nevado el
Paramillo del Cisne-Morro Negro

Dome

4700 m

4° 50' 35" N

75° 21' 8" W

Laguna, Alto de la

Dome

3850 m

4° 59' 0" N

75° 21' 0" W

Olleta, La

Dome

4850 m

4° 54' 0" N

75° 21' 29" W

Pirana, Alto la

Dome

4600 m

4° 54' 0" N

75° 18' 0" W

Santano, Alto de

Dome

3950 m

4° 59' 0" N

75° 20' 0" W

Photo Gallery

Two weeks after the November 13, 1985 eruption, steam rises from Las Arenas, the summit crater of Nevado del Ruiz. This view from the NNW shows pyroclastic-surge and pyroclastic-flow deposits mantling the summit icecap. Radial grooves descending the upper glacier surface were scoured by pyroclastic surges, which reached 5.5 km NW and NE of the summit. Melting of the summit icecap led to the formation of devastating lahars that traveled as far as 100 km from the volcano and caused 23,000 fatalities.

Photo by Norm Banks, 1985 (U.S. Geological Survey).

A record of the November 13, 1985 eruption of Nevado de Ruiz was preserved in ice-rich levees of pyroclastic flows. This photo, taken a month later, shows parallel-bedded pyroclastic-surge deposits, composed of snow grains and tephra, which were deposited over a pre-existing surface of clear, coarsely crystalline ice at the bottom of the photo. The surge layers are overlain at the top by coarser-grained pyroclastic-flow deposits. The clast of glacial ice (located left of the upper part of the ice axe) was scoured and incorporated into the pyroclastic flow.

Photo by Tom Pierson, 1985 (U.S. Geological Survey).

The unvegetated channel that descends diagonally from the right center to the lower left is the passageway of lahars in the upper Guali River drainage on the north flank of Nevado del Ruiz. Farm houses are visible on the opposite side of the channel for scale. The Río Guali lahars traveled more than 100 km from the volcano at velocities of about 5-15 meters/second.

Photo by Tom Pierson, 1985 (U.S. Geological Survey).

A cluster of rounded boulders was deposited on a river terrace by a lahar in the Río Chinchina valley, 59 km WNW of the summit of Colombia's Nevado del Ruiz volcano on November 13, 1985. The boulders were carried as bedload and deposited against the tree, which served as an obstruction to flow. Note the mudline on the tree that marks the upper flow surface of the lahar.

Photo by Tom Pierson, 1985 (U.S. Geological Survey).

Steam rises from the summit crater of Nevado del Ruiz volcano on December 10, 1985. Pyroclastic-surge and -fall deposits from the November 13 eruption extend from the crater. Glaciers on the upper-NE-flank headwall of the Azufrado valley in the foreground were scoured by pyroclastic surges.

Photo by Tom Pierson, 1985 (U.S. Geological Survey).

Nevado del Ruiz is a broad, glacier-covered shield-like volcano that covers >200 sq km. The modern cone consists of a cluster of lava domes constructed within the summit caldera of an older Ruiz. The ash-mantled summit of Nevado del Ruiz volcano is seen here from the ESE on December 18, 1985, a little more than a month after the catastrophic eruption of November 13. The devastating 1985 lahars swept down four major river drainages that day and caused more than 23,000 fatalities.

Photo by Norm Banks, 1985 (U.S. Geological Survey).

The glacial icecap of Nevado del Ruiz volcano towers above the headwall of the Azufrado valley on the NE flank. Debris-avalanche deposits of a roughly 3100-year old eruption and the 1595 eruption appear in the foreground. The major eruption about 3100 years ago produced a large pyroclastic-fall deposit, the debris avalanche, and associated lahars and pyroclastic surges. The 1595 eruption, the largest from Ruiz in historical time, was similar. Lahars in 1595 caused over 600 fatalities.

WOVOdat is a database of volcanic unrest; instrumentally and visually recorded changes in seismicity, ground deformation, gas emission, and other parameters from their normal baselines. It is sponsored by the World Organization of Volcano Observatories (WOVO) and presently hosted at the Earth Observatory of Singapore.

EarthChem develops and maintains databases, software, and services that support the preservation, discovery, access and analysis of geochemical data, and facilitate their integration with the broad array of other available earth science parameters. EarthChem is operated by a joint team of disciplinary scientists, data scientists, data managers and information technology developers who are part of the NSF-funded data facility Integrated Earth Data Applications (IEDA). IEDA is a collaborative effort of EarthChem and the Marine Geoscience Data System (MGDS).

Using infrared satellite Moderate Resolution Imaging Spectroradiometer (MODIS) data, scientists at the Hawai'i Institute of Geophysics and Planetology, University of Hawai'i, developed an automated system called MODVOLC to map thermal hot-spots in near real time. For each MODIS image, the algorithm automatically scans each 1 km pixel within it to check for high-temperature hot-spots. When one is found the date, time, location, and intensity are recorded. MODIS looks at every square km of the Earth every 48 hours, once during the day and once during the night, and the presence of two MODIS sensors in space allows at least four hot-spot observations every two days. Each day updated global maps are compiled to display the locations of all hot spots detected in the previous 24 hours. There is a drop-down list with volcano names which allow users to 'zoom-in' and examine the distribution of hot-spots at a variety of spatial scales.

Middle InfraRed Observation of Volcanic Activity (MIROVA) is a near real time volcanic hot-spot detection system based on the analysis of MODIS (Moderate Resolution Imaging Spectroradiometer) data. In particular, MIROVA uses the Middle InfraRed Radiation (MIR), measured over target volcanoes, in order to detect, locate and measure the heat radiation sourced from volcanic activity.